Yes, there is at least one explicitly nonbinary human character, plus another character who's exploring their gender! I could be wrong, but I had the feeling that the author wanted to make sure to show it as a real, human thing, and not just a thought experiment pertaining to AIs. (There are also some instances of hostility or cluelessness towards nonbinary people in the book, both from parents and from well-meaning friends, but they're pretty clearly presented as bad things, I think. As always, lmk if you have more questions/you'd like other content warnings!)

Sherlock Holmes hasn't ever been one of my main fandoms, but there is some really amazing fic for it out there. If you do read Masked Ball, I'd love to know what you think!

And yeah, so much agreement about ABA. I only really know what I've read from people who've experienced it, but that's... also all I need to know, I think.