So as far I can understand you are saying the MC's wife(even he switched bodies)is not a romanceable option(why?I like her) and you have to find her a boyfriend and watch her get fucked I don't know about others but it is ntr in my books and can you avoid it ?and have her for yourself ?
I think it's a sweet idea, actually. MC is now a teenager and he still loves his former wife but knows they can't be together in the end so he helps find her a new man.
I wouldn't mind having the former wife as a short term LI (romance her, have some closure sex with her, but then help her find someone to be with long-term), but there will most likely be an option to not actually watch her have sex with someone else (which would be the voyeur tag and not NTR because the MC is setting it up while no longer in a relationship with the former wife).
NTR is not a "some other person is having sex on screen with a character MC could potentially have sex with" tag.