RAW, The material component isn't needed if you have a focus. In the Cleric or Paladin, that focus can be, like you said, the emblem on the shield. The only time this become an issue is if the spell has a material component that as a cost associated with it, but most spells that are used in combat don't have this.

In Re: BA spells, Action Spells, Reaction Spells and the per turn limitation. The metaphysical aspect being overlooked is that with a BA spell (whether a "normal" BA spell, or a Sorcerer-induced one), it's not the amount of time it takes to cast the spell that muddies things -- it's that the mage is "compressing"/"condensing" a level of mystical force/concentration that makes doing any other spell in those few seconds at "full strength" difficult. That's why the BA/A/R scaling makes sense. (In the case of Reaction spells, they are normally even "smaller" acts of magic, regardless of level, and are super-quick -- literally less than a second, and just a small burst of power, such as Shield, Counterspell, etc. -- and acting at a degree "below" BA spells in that regard. That's why you can A-Fireball, target tries to Counterspell, and you can R-Counterspell during your turn (per Crawford's example in SA) -- the amount of "oomph" needed to incorporate Counterspell in that instance isn't as great as the focus/compression required to do a BA spell, proportionally.)

It really does make sense. (And AS is simply an exception -- a literal surge that strains beyond the normal limits -- to make "x" happen... so that makes sense, too.)

OH! -- forgot to add: the stipulation you add relative to Reaction spells gets tripped up in that there are a number of instances wherein a caster does use their Reaction during their turn to cast "x" while already casting "y" or "z". (It happens with occasional frequency in the games I/others play in with enough regularity that it definitely is a "thing".) Examples: my other post's Fireball/Counterspell-the-other-caster's-Counterspell (as exampled by Crawford); character casts spell at target, target uses a Reaction (of "x" ability) to counter-attack immediately the character, character immediately (R)eacts with Hellish Rebuke... all during the character's turn. (My tempest cleric's War Caster Feat has caused certain similar instances on several occasions via Reactions vs. Reactions within his turn.) Just saying...

Regarding spells cast as a bonus action, you state a couple of misinterpretations.

That is to say, if you cast a level 1-9 spell with your normal action, you can't cast one with your bonus action, and vice versa; you can cast a cantrip with the other action (or with both).

This comes close to explaining the rule as written but doesn't quite get it right. If you cast any level 1-9 spell with your action, you cannot cast a cantrip as a bonus action. If you cast any spell, even a cantrip, as a bonus action, the only other spell you can cast on your turn is a cantrip with a 1-action casting time. That is, your options are 1) level 0-9 bonus-action spell and level 0 action spell, or 2) level 0-9 action spell and no bonus-action spell.

That's the rule as written; whether it makes sense or not is another question.

(This even applies to spells cast as reactions between your turns - you can cast Counterspell or Hellish Rebuke as a reaction if you did the "fireball, action surge, fireball" thing, but if you cast a single quickened fireball and nothing else, you can't!?!)

This is simply incorrect. Casting a spell as a bonus action only precludes the caster casting other spells (except a 1-action cantrip) on the same turn. You can still counterspell or hellish rebuke on an opponent's or ally's turn unrestricted.

A character can only cast one non-cantrip spell during their turn, no matter what actions they have available to them. They can combine that spell with any number of cantrips that they have the actions for. This does not restrict spells cast not during your turn, such as a Hellish Rebuke cast as a reaction.

I read this as a straight nerf to any player that might want to cast a reaction on his or her turn. Someone might want to use absorb elements, hellish rebuke, or shield in response to an enemy's attack of opportunity, for example, or feather fall or soul cage in response to the caster's own actions. It hits wild-magic sorcerer especially hard, since a number of the results on the surge table are spells that the player might want to counterspell.

Apologies, made a small typo. Instead of "or 2) level 0-9 action spell and no bonus-action spell," it should be "or 2) level 1-9 action spell and no bonus-action spell."

Clerics and Paladins get focuses for their shields because they are support tanks. The use of shields is built into their class and purpose. Eldritch Knight doesn’t get a focus at all. The Wiz and Sorc don’t get shields without MC or Feats, and Warlock only gets one with Hexblade, which is from a later book.

The idea is not to require a feat for two classes to do what they were intended to do, but require it for the ones that are not intended to do it.

Wow. I know the PHB phrasing over bonus action casting was clumsy, but you made it even more complicated and confusing. Try this for size -

When you cast a spell using a bonus action (by listed casting time or if quickened) then that places a restriction on any other spells cast during your turn. That restriction is that they can only be cantrips with a casting time of 1 action.

That's it. Simple and clear. You can use Action Surge without a bonus action spell to cast Fireball twice if you wish. You can cast reaction spells in your turn if you don't bonus action cast. But once you bonus action cast the restrictions come into play for the whole of your turn.

The metaphysical aspect you are describing is a load of bullshit. If you can compress a mystical force in such a short amount of time you have more time for a regular full strength spell. Why would that be more difficult than casting a regular full strength in less time?

Bonus actions aren't just faster, they're also more complex.

Bonus casting is like trying to ritual cast in 6 seconds. Sure, the spell is completed more quickly, but it takes more of your concentration to do so.

You might have more TIME for more actions, but not enough FOCUS.

Casting as an action or a reaction implies that the spell is simple enough to be cast during those 6 seconds without straining your mental resources.

It also becomes an issue if the spell has somatic components which require a free hand to perform. You can cast Healing Word with just a shield-emblazoned holy symbol and a mace in hand, but not Cure Wounds - you'd need to drop, holster, or pass the mace to your shield hand.

This feels mainly as a fluff describing mechanics that are needed to ensure a balanced game. Also, the scale of Counterspell for example can be debated - casting it at 8th spell to counter some cataclismic spell feels really strenuous to me.

I really don't think this needs as much explanation. It's far easier to just say that casters can't use more than one slot per their turn, with reactions being the exception. If you dig into it, it doesn't make much sense (e.g. the Counterspell), but the game needs this on a mechanical level.

not enough FOCUS? Yes, you have. Otherwise you would not be able to cast a spell as a bonus action in the first place.

(Just jumping back in after a long absence)You're overlooking another aspect of what's being addressed. It's not just a question of time, but again of compression/exerted power. The "twist/turn" of a BA spell makes doing a "full strength" spell metaphysically impossible in the same few seconds per the metaphysics of the game/setting. Think of it this way: you might be able to take a shower (Action spell) while turning on the sink faucet in the bathroom (Reaction Spell) without having a major impact on the shower, but flushing the toilet (BA spell, in this case) means something has to "give" relative to the shower/sink situation. The shower loses pressure (can't cast a level spell during those seconds the toilet is filling), and you can only just run the sink so as to brush your teeth/wash your hands, etc. (smaller acts of "magic" -- Cantrips -- that can work "properly" while the BA spell/toilet is happening).

That's how that works.

In Re: Counterspell. Part of the issue that is also misunderstood (though this is understandable as it isn't as directly explained, or at least "loudly" enough, in Sage Advice and similar)is what Counterspell is/does. Counterspell isn't a great expression of mystical power... rather, it's a very finessed one. Counterspell doesn't stop another spell by being a sledgehammer/tidal wave of mystical force, but rather as described it attempts to snip the mystical threads of a spell as it is forming and thus prevent it from manifesting.

Think of this way: a highly advanced car (high level spell) may be able to do a number of things and/or be very powerful (high speed, sharp turns, etc.), but if just one or two certain things go wrong with the engine (and particularly the computer operating it) then the whole thing can shut down/"fail". Counterspell isn't a giant boulder come crashing down on the car -- it's a small/finessed tweak of a given part of the engine or a single chip in the computer. A flick or crack here or there and the whole thing comes to a halt. That's Counterspell. Casting it at a higher level isn't so much pouring more power into it (though there is likely a degree of that) but more so that the caster is casting it with a level of skill/finesse (reflected by being high enough level to cast it at "x" level) that they can counter automatically appropriately higher level spells but at the cost of calling upon their higher level energies or limited super-focus.

In Re: Focus and BA spells. But that's the whole point -- the greater degree of focus/what-have-you is temporarily channeled into the casting of the BA spell. There isn't enough focus/compression, coupled with power, to cast a BA spell and a non-cantrip Action/Reaction spell in the same few (usually less than six) seconds that represents a character's "turn" in a given "round".

Once you really take a step back and realize that what is being described is taking place in a period of a scant few seconds, it really does make a great deal of sense.

The problem isn't in-universe justification; a clever person can make up reasonable-sounding explanations for any coherent set of rules.

My whole complaint is that it's not a well-written rule in the first place. It ends up with confusing non-obvious implications, that could be fixed if it was written in slightly different ways.

Your first paragraph is chalk full of errors.

You can't cast a bonus action cantrip if you cast a leveled spell as an action.

Turns and Rounds are different things, so if you bonus action Healing word, then on another creature's turn it casts a spell and you want cast Shield or Counterspell you absolutely can. You just wouldn't be able to use that reaction on your turn [if you say provoked an AOO and wanted to shield if you've Healing Worded].

You're just completely off base so hard that I don't even know how to correct it on the spell casting component part.

If a spell is M and S you can have your hands filled as long as one has something you're using as a focus for the Materials [and the materials the spell aren't consumed in which case you'd have to provide them].

If the spell is just S you can't have your hands filled, even with your spell casting focus, which granted is a little bit odd but that's one of the big features of the Warcaster feat, as well as one of the Xanathar's Warlock invocation options.

Divine Casters get a part of the feat, only for spells that are both S & M, if they have the symbol on their shield, but arcane casters can do that too if they have their focus affixed to a staff or wand. They just can't have it affixed to a shield [which isn't a big deal anyways because they can't generally wield shields].

I suggest you hit the books.

Well, I had cited similarly in my series of responses (granted, not in the specific one you're citing,#15, but in #14), vis-a-vis how certain things are explained. (You have to piece together different bits from the books along with some SA responses, which is of course not an efficient way to cover those things.) What I've responded, though, over several posts is sound relative to the game/metaphysics as-presented. It really does make sense when the examples are read through and contextually understood. (Again, admittedly, this all should be addressed/explained better within the books... but it isn't unsound once it's properly understood. It really does make quite a bit of sense -- it's just not explained in a single spot/place, unfortunately.)

Exactly. To add a couple things to support your post:

In Re: S/M spells vs just S spells -- the reason/underlying rationale for how the same hand can be used for the M plus S in the one spell but you can't have your hands full for a just S spell is because the degree of S needed for an also-M spell includes manipulating the M in question (holding M and making a quick couple of swirls in the air, or making a symbol via gesturing with it, or similar). The S in that regard isn't what it is for a V/S or just S spell (which is implied to have, even if very quick a la' a Reaction spell like Shield or Counterspell, a degree of finger movement that isn't readily possible while holding something). As you cite, the feat War Caster includes the caster having learned how to channel the power/gestures via objects held (weapons, what-have-you) so that is a contextually exampled exception around that.

In Re: turns vs rounds. This is also where a degree of abstraction does legitimately have some/many players a bit confused -- and understandably so. Crawford/SA has explained in a few different posts that it is rare for a given character to being acting throughout a given 6 second round; it is RAI/understood that the character is only acting for a portion of that time, with the other bits of time representing their responding to things in "non-acting" ways (head turning, pausing for a second here or there to readjust split second perspective, ducking/stepping a bit here or there to avoid things (which is what AC and some Saving Throws represent), etc...). The tricky thing for many to fully understand is that the Initiative order doesn't directly represent a point where a given character gets to act for a full six seconds but rather when/how their specific series of core (A)ctions occur/fall over that time relative to the "post-production editing" of everyone else's (A)ctions. That is, it isn't that a given person gets to "do all their stuff", so to speak, over several seconds while all the other participants stand around in doing the video game-esque "back-and-forth shuffle", but rather their "initiative" represents that a key points over those seconds are when their (A)ctions (Action, Movement, possible BA) have "priority". In the post-production edit, all the positioning/movement is spliced together to represent when "x" and "y" and "z" are happening in an interacting sequence. That's why when it's "your turn", you have a given metaphysical limit of how much "juice" you can channel via the spells-as-presented via A/BA/R(that last when it's during "your turn", vs an R when it's someone else's turn). It sounds tricky, and it can be for those who are thinking about it all a bit too linearly, but once the mental click happens it really all comes together.

One more thing to note about the player handbooks wording of casting two spells on the same turn is that, because a cantrip is a spell, and casting a spell as a bonus action prevents casting a non cantrip spell as an action. The quicken spell meta magic on a sorcerer or the 2 bonus action cantrips, magic stone and shillelagh, prevents you from casting a spell as an action that isn’t a cantrip even though you didn’t spend a single spell slot this turn! So a sorcerer thar wants to cast a spell and a cantrip and use their Meta magic twice in a turn can’t quicken the less potent cantrip and twin or empower the spell, they have to quicken the spell and use the cantrip as an action. Meaning sorcerers who try to nuke waste a bit of sorcerery points twinning cantrips instead of twinning chromatic orbs or something more valuable.

This last example has TWO reactions "all during the character's turn", which is a no-no (the counterspell to the counterspell, and the Hellish Rebuke).

What about casting a reaction spell during the same turn as you cast a spell on the action? Casting Counter spell to counter another caster's Counter spell of your fireball? That's 2 spells in one turn, 1-fireball and 2-counterspell.

My first, simplest version of the rule would disallow that (one non-cantrip spell per turn). My second, slightly more complex version (one non-cantrip with your action or bonus) would allow it - other things like reactions are unrestricted.