Is it always valid for someone to take an otherwise offensive notion and apply it to themselves?

I think, yes. But only as long as you understand just because you might be ok with a word, doesn't mean other people who are part of that group also are. I mentioned in another comment that as a cis-het white male I don't think it's very productive for me to go up to a minority person I hardly know and say "hey I heard you saying fa**** and that's offensive you should stop" because first of all it makes me seem like a self-righteous prick and also it's just divisive. If they're a friend or their usage is especially problematic or excessive I might try to explain that that term can still be offensive to other people but otherwise it's whatever.

However in the context of mental illness, let's say someone with anxiety has trouble during during finals and they talk about "Lol I can't study because I'm craaazy". Even if they are being light hearted about it, "crazy" isn't exactly being reclaimed the same way that other terms have been and is basically being used in it's original pejorative context. I worry if "improper reclaiming" by a minority ingroup helps to normalize the word's usage by the majority group. (I just made the phrase "improper reclamation" lol).

It's like how Benedict Cumberbatch got criticized for using the phrase "colored people". I mean he was addressing problems of racism in the casting process in the TV/film industry so I highly doubt he meant to be offensive. He possibly had heard it before and didn't realize it was problematic and probably didn't think it was much different that saying "people of color".