okay so. idk if this would work for anyone else's situation, but refresher on what happened with lily for me: as far as i can tell, she came from my friend's void after someone else moved out--completely naturally, no amiibo forceouts. the following day her plot was being built as normal but then the next day after that she was nowhere to be found, with her house being there but simply saying "i've moved out - lily" on the door.
i've been really afraid to try pushing her out with an amiibo because i was worried about glitching my game even more, but my brother who i share my island with said i should just go for it. so i replaced her with snake and now snake is successfully on my island where lily's glitch house used to be! for anyone who something similar has happened to, if you have amiibo cards this just might be what you have to do as nothing else i tried worked and i was tired of waiting for it to be patched. i tried a lot of TTing, complaining to isabelle as someone else suggested, and even moving her house but the option literally wasn't there for me. trying an amiibo forceout was all i could do and it seems to have worked.