My partner has DID, and I've done a ton of research into it as a result. This story sounds extremely plausible to me.

I've read multiple case studies where people with multiple personalities will get out of whatever situation was causing the disassociation, and over time some of the personalities will vanish / die off. There was also a very extreme case I read about where the fractured personalities managed to coalesce into a new whole, but it was a different personality than the original. Basically a fully formed identity that was suddenly living the life of someone they didn't identify as, and whose memories they couldn't really recall.

Even in my partner's much less severe case of DID, the less prominent personalities will sometimes go dormant for months at a time. Haven't had any of them disappear fully yet, but it's at least theoretically possible from what I understand.

Sounds actually spot on for depersonalization disorder
https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/dissociation-and-dissociative-disorders

I've watched an interview with someone who has multiple personality disorder and it was really fascinating. She didn't tell what happened, but she must have had a really fucked up childhood, her whole arms were covered in self harm cut marks.

My understanding is that as a defense mechanism the personality splits into two (or more) parts and one takes the suffering so the others can live an otherwise normal life. So anytime she was abused the same personality would be in charge, making sure that the other ones do not have to experience that trauma.

There is probably a lot of complicated science too.