Yeah last I heard Xavier exists on the astral plane with the Shadow King.
So eventually a new body will be prepared for him and he will download himself into it.
As to Mar-Vell, perhaps he has reached the point where he no longer likes being dead and all the recent upheavals with the cosmos have prompted him to return.
Agreed. If Mar-Vell comes back, It should be a big deal.
Eternity and Death should show up and agree that the universe needs him.
And yes I know all about the weird stuff going on in ultimates.