How does resetting quests for a guaranteed legendary interact with multi-player?

If you reset your quests and play the game all the way through to Diablo and kill him, you will get a guaranteed legendary from his death. That, when playing alone, is simple enough to understand.

But how does it interact with other players who may or may not have also reset quests?

Here are a few examples:

1. If Player A resets quests and Player B does not, and they both play through every quest together, does Player B get a guaranteed legendary?
2. Does the answer to the previous scenario change if Player B only joins during Act 4, and doesn't play through himself?
3. If Player A resets quests and joins a game in progress from Player B, who did not reset quests, in act 4, will Player A get a guaranteed legendary despite skipping 3 acts?
4. Does the answer to the previous scenario change if Player B had reset quests and got this far on his own?

What are the exact conditions required to get a guaranteed legendary?

To get the legendary to drop, the kill on Diablo must be the first since resetting all quests. Since loot drops per player, and not for all, playing in multiplayer guarantees the legendary for any player in the party that meets this requirement.

If you reset your quests then join a different game in act 4 and kill Diablo, you should get a legendary for killing him. However, since you didn't complete the first 3 acts you won't be able to reset quests again for another legendary. This means it's best to just go through all the acts after resetting. You will have to go through them all at some time or another anyways.

tl dr:

1. No
2. No
3. Yes
4. No

You don't actually have to play through all the quests to get the guaranteed legendary. The guarantee is simply that the first time you kill Diablo after a quest reset guarantees a legendary drop. The reason you have to do all the quests is to be able to reset them again.

This drop doesn't depend on anything else. I've both carried and been carried for the Diablo kill and the leg always drops if the condition is satisfied.

There is one caveat. The game treats every character's first Diablo kill after patch 2.0 came out as a fresh kill and will guarantee a legendary. If you killed Diablo before they gave him the guaranteed drop but after patch 2.0 came out then you will not be guaranteed anything. Previously it was the skeleton king that guaranteed the drop but it was moved to Diablo.