As far as contact with your dad, can you call him up to meet him for lunch. Could you be very honest and tell him that you would like to see him and that you miss him, but that you can’t have your mother in your life? Or is he so enmeshed that this would never work? While my father was alive, it was very clear to me that my mother was so enmeshed that if forced to make a decision like this she would have chosen him, at any cost. I would caution though, only do this if you are prepared to handle the pain it would cause if he chooses not to see you because of your mother. I suspect he may be too enmeshed.
You said that M still wants to see her. I know this is hard. I don’t want to drag my kids into my problems with my mother. She does treat them differently than she treats me, to some extent. But, after what she has done to me, I don’t trust her to have my children’s welfare as a top priority over her own selfish needs and melodramas. I have had to make the choice for my kids. I pray that they don’t hate me for this someday, but it is a risk I cannot take.
You wrote about M’s father:
He has done some reading about mild a-spectrum disorders, but he thinks they are all bunk.
This jumped out at me as a very pink flag. They are not bunk. Even with mild a-spectrum disorders, the brain processes information differently. Before I would let him near M, I would make sure he understands this very clearly. I have a relative age 7, with PDD (mild autism). He sees the world differently than most kids, and interacting with him requires that I adapt to how his brain works. I cannot expect him to meet me where my brain works. (This is true for all kids IMO.)
The only other concern I would have is why hasn’t he attempted to be a part of M’s life. The grandparent’s as well. Maybe you can meet with him and his family (a number of times) without M there and then make a decision. It would be wonderful for both you and M to have someone to rely on. And it could enrich M’s life. Again though I hate the thought of putting M through any upheaval if it doesn't work out.
Peace
PS - I just read your post, you said: I want to believe in the basic decency of people, but when it gets right down to it, the only person I really trust is myself. I was this way too for a very long time. I believe it comes from never having anyone to rely on as a child. I think the only way to get past it is to take a risk. But, make it an educated risk - learn everything you can about this person before you trust him with you or M. Given that a child is involved, I would go so far as to have a background check performed (see how suspicious I am

)