Also, would it have been possible for Marth to pull a Lyon and bring Cornelius back so that Dad could deal with that dragon problem?

This is a really interesting - albeit more than a little disturbing - possibility.

I guess the same could be asked in Elibe, too - do they just cart Ninian's body along with them back to the Dread Isle? That really can't. . .keep well. . . . >__>