If there is a situation where there are multiple kills and multiple saves and cancel options, can the saves and cancels occur in any order

If player A saves a character and then player B / C pass (keeping the cancels for other character saves), if player A also passes I assume that would be then end of the save/cancel window and we could not go back and cancel the save

Correct.

Also true for cancelling the thing your saving from (eg. Outwit)

Player A passes, expecting C to outwit so not wasting his saves. Player B passes, as he has no saves. Player C decides to pass, condemning all of player A's characters to die.

And this is why I'd really like to see Saves and Cancels as two completely different Response windows. The fact that you can save from an effect that can still be subsequently canceled is a bunch of cockapoo.