Is it permissible for someone whose spouse is presumed dead to remarry? If so, what would that person’s obligations be if, after they had started a family with someone else, their original spouse turned up alive?
PROCESS IN THE PRESUMED DEATH OF A SPOUSE
Can. 1707 §1. Whenever the death of a spouse cannot be proven by an authentic ecclesiastical or civil document, the other spouse is not considered free from the bond of marriage until after the diocesan bishop has issued a declaration of presumed death.
§2. The diocesan bishop is able to issue the declaration mentioned in §1 only if, after having carried out appropriate investigations, he attains moral certitude of the death of the spouse from the depositions of witnesses, from rumor, or from evidence. The absence of a spouse alone, even for a long time, is not sufficient.
§3. The bishop is to consult the Apostolic See in uncertain and complicated cases.
If the presumed-dead spouse turned up alive, the people involved should consult their pastor and bishop for guidance
Wasn’t this what happened in Castaway?
I don’t know why they would need guidance. If the first spouse is still alive the person is still married to them and at the very least would have to stop having sexual relations with the new spouse because now it certainly is adultery and a mortal sin.
They weren’t married, he was going to propose before he became a castaway!
Correct, we could swap out Castaway for My Favorite Wife (Cary Grant and Irene Dunne) or Move Over, Darling (James Garner and Doris Day).
It’s an interesting scenario to ponder!
Of course they would need guidance. It would be a shocking and difficult situation. The OP mentions having a family with the second spouse in their hypothetical. All parties involved would need counseling, IMHO, for the best course of action for all parties involved including children.