Okay, I'm not a tactful person, so thanks for not being offended. ^__^
Honestly, this really, really sounds like mental disorder. I know a lot of people shy away from those words but they happen. I've had several (currently only my life-long one) so sometimes I forget how much people can be freaked out by that possibility.
The affect, differentiation/dissociation with multiples and shapeshifting belief, confusion (about the doors), and migraines kind of seem like a dissociative identity disorder. It kind of sounded like the "sides" were a part of her, you know, like multiple aspects of herself. This disorder is *extremely* difficult to self-recognize and you don't need all the symptoms, which is why I recommend the therapist.
Also, it's not even for sure that the issue is this. I took a test to see how dissociated I was and I scored pretty darn high, but I don't have this disorder. I'd just, like I said, get it checked out with the idea that the issue is some kind of dissociative problem.
There's nothing here saying she's not otherkin or she can't be otherkin. I'm not saying she isn't, I'm just saying that the very limited number of symptoms you've presented seem more along the lines of a psychological issue. But, I'm not always right (unfortunately
), so she should still keep her options open. It's more likely a mental thing, though, instead of otherkin, and I'm sure she wouldn't want to jump to incorrect conclusions.
P.S.: NEVER self-diagnose a psychological issue, that's why I kept recommending a therapist. Trust the professional. At the least she can rule out the possibility of a mental cause.