But that being said, why wouldn’t our case be comparable? She is permitted to remarry (since she has positive evidence that her husband died) but if he should somehow return (say via Techias HaMeisim) then her children by her second husband are mamzeirim.
Wolf: You are equating BD allowing the wife to remarry to Hashem allowing her to remarry. That is just not true.
In the case of Ba’ah Memedinas HaYam, as far as Hashem (who is omniscient) is concerned, she is not allowed to remarry. The fact that she can return to her first husband is only due to her being an Ones. As far as Hashem is concerned, she is only married to the first and hes never been married to the second.
As opposed to the case of Techiyas HaMaysim, as far as Hashem (who is omniscient) is concerned, she IS allowed to remarry. When her first husband gets up (let’s say like R’ Zeira), she will Halachicly be married to both. (And the concept does exist in Halacha, see Gittin 82B, Keddushin 60a).
WADR I think this is where it gets confusing: Mamzerus is only created by Biyas Issur. If a woman has two husbands B’heter, then a Mamzer can not be created.