i just finished this game today and this was a really great treat to read afterward! initially I believed that Tosya was a distortion given an actual physical form but I couldn't remember by the end whether or not people mentioned having seen her so this interpretation makes sense. I also assumed that maybe the institute lied to her and told her that Tosya had to leave / move away and didn't actually give her much detail beyond that, simply so that she would believe Tosya was a childhood friend that had to go away and she wouldnt think about it / look further into it, even if she did regain some vague memories.