I disagree on the "problematic" front for otherkin/therian. If you did something bad in your past life, and that is something you talk about/have written anywhere, then you'd probably still be labeled as such. But you are right about the unique fear of fictionkin having doubles-- if these kids are looking to be unique, well, this isn't the way to do it (I say this as fictionkin myself, though I fully accept the fact that doubles do exist and no one can "stake claim" to an anime character or whatever).
I also agree with your second point-- I wish I could do more to discuss old otherkin and more grounded communities, but I've never been able to be a part of one.

Very plausible, and admittedly, I wouldn't be surprised if the offshoot of otherkin, fictionkin, started out as a joke to mock them. Fortunately or unfortunately, it actually did resonate with some people (particularly those who used their favorite game, anime, etc. to cope throughout life, but in other situations as well). I also see a high rate of autism mentioned in both communities, though I have no idea if it actually plays a part (I myself am autistic, and all of my various-kin-type friends and many people I've crossed paths with seem to be, too).

Pack mentality, how ironic. I feel like this falls right back into the high school thing; it's better to pretend to go along with the bullies than actually stand up to them, because they're afraid of being cast out/made fun of. Which, as mentioned, is a horrible way to live.

Hell, as far as kin goes (and I'm sure you can tell), at this point I don't even know what's normal. I could blame tumblr again, but it's also in part of having no "in" to the actual community.