Oh DM, I'm so sorry. What an awful situation to be in. You have put up with fear and abuse for a very long time while doing your utter best to help her. I understand and support your choice.
This does seem to be the bottom line, doesn't it? S loves playing the victim, too. Perhaps sympathy is the only kind of attention they understand. There is a deep-seated need there to be the center of drama and to play to victim role. I don't know why.
It sounds like you are still committed to connecting her with services that can help her before stepping away. That is very commendable and brave. Are you planning to tell her you are going no contact after this? And why? Or do you not feel like it would be safe to confront her directly at this point? Are you planning to move?