Haven't seen this topic addressed with respect to the 5e rules, but there are a couple of issues that require clarification.
I'm happy to clarify the rules for you, but they really are clear.
When it is your turn, you do everything you want to (and can) do, before your turn ends and it is somebody else's turn. The rules never assume you complicate things by mixing up two different creatures' turns, so I suggest you... don't
As fasr as I can see this pretty much negates all your questions, as far as the RAW angle goes. Feel free to repost if you still feel something is unclear.
Otherwise, I suggest this discussion is continued in the homebrew forum, since everything regarding your thoughts is definitely houserules: you simply never split up a turn in the official rules.
Regards
Zapp
2) If a creature with multiple attacks kills/knocks out an opponent with fewer attacks than the total it has available, can it switch the remaining attacks to another creature? (I guess another way of asking this question is, how many people use the Cleave-Through rules from p. 272 in the DMG, especially against PCs?)?
No no... you are free to move between any of your attacks in 5th Edition. So if you can reach another foe with any remaining movement, you can use your remaining attacks on it, no optional rules needed. (If you're an archer this is often trivial).
Move up to Goblin A
Attack Goblin A. Goblin A dies, but you have a second attack left.
Move up to Goblin B
Attack Goblin B
Very easy
3) Does the massive damage rule apply to multiple attacks if all taken in a single turn from a single source? (E.g. a 5th level fighter with 2 attacks is fighting a goblin with a total 10 hp. She hits him twice, doing 10 damage each time. Has the goblin sustained a massive injury (and is therefore dead), or does the fighter have to do 20 hp damage with a single blow to have this effect?
Single blow. Never add up effects of different attacks, even if made using the same attack action.
4) Does anyone break up the initiative of multi-attack creatures/characters as a house rule? E.g. roll three separate initiatives for a troll with two claw attacks and one bite? Obviously, this would be undesirable as a regular rule if you have lots of combats and combatants, but it might be warranted if a party faces a single creature with multiple attacks. Things won't bog down, and it would eliminate potential problems related to question 3). Anyway, that's kind of how it worked in earlier editions (if you used weapon speed).
Best taken to the homebrew forum.
