To your other point, I don't think anyone thinks "negro" when they hear, see, or use the term "n-word".
either version of the n-word
I don't know if that indicates discomfort on your part, but I definitely am uncomfortable with saying either. Even though I recognize one is more a pejorative than the other. Of course,
mutants have never had a problem referring to themselves as mutants
I don't care enough about MU to say whether or not this is true. I will say that the only people I knew in person who used the "n-words" were black (and, you know, racists). I also wasn't around for the transformation of the "n-word" from an "acceptable" descriptive pejorative into something it almost causes me pain to say and is unthinkable to write. A competent or thoughtful writer could be taking their readers on a trip through the wayback machine... but I'm not the one who's going to have that optimism dashed, so...