Christine reading through your experiences, it's amazing how well you have handled everything that happened with your mom!
I think many people would have gone no contact after such unpleasant experiences, but you stayed in contact, realized she had been projecting her needs onto you, figured out how to meet her needs, and helped her to accept your help. I'm amazed that you were able to do all of that in such a charged situation!
I'm trying to figure out what you are saying about masturbation. Are you saying that you have never discussed it with her, but during her breaks when you visit, she is able to be alone without the belt, so she has opportunities to masturbate alone if she wants to?
And you don't care, and are fine with it, as long as it's not with one of her boyfriends?
Does she spend much time alone without the belt when you visit? For example is it normal for her to be in her bedroom or the bathroom with the door closed for more than a couple minutes?