Asklemmy

The oldest quicksave point takes precedence. Nobody actually experiences anything until the player with the earliest quicksave establishes a new save point, or otherwise becomes permanently incapable of restoring that earliest point. Whatever was experienced between the oldest and second oldest quicksave then becomes the unalterable historic record. Everything else is an aborted timeline that never actually exists.

I just thought "hur hur, Nazeem" and save scumming skill checks, dice rolls and tricky input in mostly singleplayer games, without any nasty precedence or concurrency issues. Extending it to multiplayer and also being inside the game seems, uh, complicated. I'll give it an undercaffeinated try:

Each player gets an individual "marker" they can place at their current time, and a function to restore the entire universe state to that point.
"Whose marker is when" seems like it needs to be part of that state. Otherwise, reverting and then having someone else reload a formerly earlier, now future/orphaned state... just sounds like a clusterfuck. Or it's unproblematic and just weird, I'm not sure.

Keeping memories across reloads would at least not happen "naturally", since everyone has their exact brain state reverted. You could just say it does for the purposes of the experiment, but it seems like it makes things more complicated.
At least, remembering stuff through someone else's reload is right out: everyone on the planet quickly ends up with a bunch of memories that have no longer happened, and no way to tell what's what. Psych horror time!

Whoever saves first does get to revert everything since then, but assuming no memory retention, you could still safely shit talk your boss all day long, at least. If their checkpoint reverts yours, they will forget the rant, you can still revert. It would be further back than you intended then, but you would be blissfully unaware of that fact. Of course, you also wouldn't remember the rant, so it doesn't sound very cathartic either.

But, if memories are retained, Boss could reload on you - they now remember the rant and you don't, which sounds like a bad Christmas Party. While reloading would still be a win for you, you wouldn't know to actually do it, and could risk saving at a position where you've screwed yourself. Common risk of save scumming.

I would assume there is an order of operations to the madness. And quicksaves are stored globally, so whoever quicksaves first is able to undo the later quicksaves. In this scenario, if your boss quickloads before you do, then they would retain their memories and go back to before the meeting knowing you were going to insult them in it before you even did the first quicksave.