Shmoel: Once again, there are a lot of factors involved that I (or anyone here) is far from qualified to judge. And these questions have to be far more detailed. For example, if there were Eidim that actually said that he died then she might even be allowed to remarry him. But each case like this has thousands of potential Tzdadim and really requires looking into it deeply.
(I had a whole Shtickel written out on some specific cases but I erased them. I am not confident enough in anything I say here to actually post it because I do not know if it’s Emes and do not want to give even the slightest misinformation on a topic like this. Not because this type of topic is more important than any other Halachic topic C”V, but because things like this are so complex that if I were to make a mistake (as happens often enough) it would not be so easy for someone to point it out by just bringing a simple Mareh Makom.)