Those are both extremely easy questions to answer:

First, if your characters die they DO come back, with time, the game will let you know by saying "Spider Man is restored" or "Deadpool is restored", something like that. To revive them, go to a S.H.I.E.L.D. save/check point and choose "Revive Party" and it will bring back any of the members who are fully revived.

Second question: Yes you can replay any mission as often as you want. Go to a S.H.I.E.L.D. save/check point and choose "Extract" now it will let you choose what part of the mission and what mission you want to play through again!

yep.
wait for four minutes then go to a S.H.I.E.L.D point and get them back

and extract is a travel option.you can go anywhere you went.



both of them requires a S.H.I.E.L.D access point.in other words:his two answers simplified