YOu are very well spoken. If in your shoes, don't know if I could keep it all in either. There's alot you need answers to & to get off your chest. She/they may blow you off but at least you tried & they know how you feel. Now if you choose to stay away on a long term basis they/she will have the letter. Whether she keeps it, doesn't agree, or keeps making excuses is up to her. You've already talked to your dad & he said he didn't care, well there's one side you won't have to guess about.
Who knows how your mom will react, I suppose be prepared for anything.

You are moving out, just getting out of such a toxic environment will help. You never know how parents will react to that, maybe not at all. You are an adult, do what's best to help you move forward with your life.

Take care & keep posting.