I'm not sure giving the nephew the ex-MIL's info was a good thing or not. Is he the one who should be telling the MIL that he's her grandson, or is that likely to hurt the 2 of them more for him

Maybe not, but I think LW is excused from having to do any of the emotional labour in this situation. I can well understand that she'd want to just hand over whatever information she had so that she could be uninvolved for however long she chooses, and managing other peoples' hurt is not her responsibility right now, even if she loves (some of) them.