I've not read the whole thread but I wanted to say, I have had similar experiences and I'm very sorry for what you've been put through. Sadly I identify with a lot of your post.
If you remember nothing else from this message from a stranger, please let it be this...
the shame that you carry has nothing to do with you. It was placed on you by sick people who wanted a convenient excuse for acting like monsters. Your parents are/were ill in the extreme, and did nothing to help themselves. Instead they chose a small girl to turn into a scapegoat for all the horror they pent up inside themselves. There is no shame for you. None at all. It's an illusion they planted in your heart long ago, for the express purpose of using it to control you.
For many many years I carried shame around with me. It coloured all my relationships, all my interactions. I felt like I had no right to walk the earth, that nothing truly belonged to me, that I deserved nothing and should be grateful for what I was given.
It affected my marriage hugely. But five or so years ago, I sat up one morning in bed after we'd had an argument the night before, and I'd been lying awake thinking how horrible I was, how ungrateful I was, and I said to DH
"You know what, none of it was my fault. None of it. It was my parents' fault. There was nothing I could have done. If they had been better human beings, everything would have been alright. It was all their fault. It had nothing to do with me at all"
That was a turning point for me. I felt the full force of rage for them that day but I was at last free. I could look back on all the events in my life and look at my younger self with sympathy and love and pity. Instead of pulling apart my every action in retrospect, arguing with myself that I probably asked for it.
Now when someone asks me about my parents I can say "they were abusive and horrible. It's wonderful to be away from them." Without wincing or flushing or getting tearful. Because it's the truth, and it may be sad, but it's no reflection on me.
If I were you, I would write a generic letter to all family members, outlining the abuse (on a very high level -- no details necessary) and saying this is why you will not contact your mother, and close by saying that anyone who contacts you about her, or on her behalf, will be subject to legal action. Send a copy to your mother with a note attached explaining that you've sought legal advice and will not accept any more correspondence from her.
The law is on your side. Your parents were and are criminals. The police will believe you if you choose to talk to them.
Much love to you x