I've been away for a few days, and I've just spent the time to go through 200 posts or so. One thing jumped out at me...
One thing about how players seem to think 5e works just astonishes me, and I believe stems from 5e's lack of wording; something that players of 3e would not do...
...and that is: not realising the consequences of the 'instantaneous' duration!
If something has a duration of 'instantaneous', then there is a 'before' that instantaneous event, there is an 'after', but there is no 'during', because 'instantaneous' denotes an infinitely small (but non-zero) period of time. If a duration can be subdivided, then it cannot by definition be 'instantaneous'.
So spells with an 'instantaneous' duration but with multiple beams/attacks, it cannot be that you could resolve the first beam, have your character wait to see if this kills the target, and then use the information to either attack the same creature with the second beam if it is still alive, or switch targets to attack a different creature with the second beam if the first is dead.
This is because the observation of the results of the first beam must occur after the beam's instantaneous existence, and by that point in time the whole spell and ALL its beams has come and gone.
From these forums it appears that many 5e players play it as if the spell had a duration of '1 round' during which you have several beams to use. This astonishes me.
However, multiple weapon attacks are not assumed to all happen simultaneously. In most cases it would be impossible for them to occur in the same instant, but you could say that you could make two weapons hit at the same time, and I suppose you could fluff that two attacks with a spear was one spear thrust going through two enemy bodies...
On the larger topic, when it comes to the actual rules of the game, they are permissive: you can only do something rules-wise if the rules say you can. You cannot say, "Ah, but nowhere in the rules does it say that a 1st level barbarian can't cast 9th level spells, so I can!"
(BTW, this is in contrast to non-rules things, like breathing or shaving or eating. You don't need a rule to give you permission to do non-rules things)
Does this solve the dispute between, "It doesn't say that actions are divisible" versus "It doesn't say that actions are indivisible"?
Yes.
If we restrict our rules actions (like 'attack', 'move', 'cast a spell', etc.) to only those which the rules specifically allow, then:-
* I can attack, move, and attack again later (I have Extra Attack and I rule which says I can do this
* I can cast misty step (and I can cast it whenever I want during my turn because I have a rule that says I can
* I can do them both in the same round, because the rules say I can take an action, take a bonus action, and move, in my turn
* therefore, I can attack, move, cast misty step, move, and attack again, because I have restricted all my game rule elements only to those things the rules say that I can do
* given that I am only doing those things that the rules say I can do, and since that sequence of events is not absurd in the fiction of the game world, the only thing that could disallow this is...a rule that says so!
* since there is NO rule which prevents me from doing those things in that order, and there ARE rules which allow everything I've done in this round, then I can.
QED
IF there were a rule which prevented this, then it would....prevent this. But there is no such rule.
If there is, cite it.