I hear you fam. It's hard to reconcile the good, loving people we used to know as capable of doing monstrous things.
Just don't blame yourself for not noticing if this turns out to be the case. I only have this perspective because I myself am autistic and have been around other autistic people of varying levels of severity, and later in life got to learn about how these sexual predator monsters operate. Someone like A makes the perfect victim for these monsters. Socially isolated. Non verbal. Easily manipulated.
Most people know fuck all about autism even today, never mind 20 years ago. It is the kind of field where if you were to Google about it, you're more likely to find Facebook moms spouting bullshit theories than actual scientific fact. Even the likes of Autism Speaks is very much guilty of this. Now dial back the clock 20 years and you had even medical doctors spouting patent bullshit.
It wasn't much better when it came to the behaviours of sexually abused children either. That shit just wasn't in the public eye and as for professionals...it's safe to say things have come a long way since the early 00's.
So yeah, there is absolutely zero chance you could have figured out what was going on. By the sounds of it, even the adults around you didn't, and I can't even blame them for missing it either.