I had to sort out a lot of my ambivalent feelings before I was reconciled to a new relationship with my mentally ill mother.
First of all, are you still struggling with any loss with your mother? For instance, grieving the loss of a normal childhood or the loss of natural mother-daughter kinship? I was very angry with my mom for getting sick on me and not being available during my adult years. If you have similar issues as mine, work through those feelings first, and maybe some of the current anguish and tug-of-war will dissipate.
I don't bother addressing the manipulative/sick part of mom anymore. I ignore that woman, hang up on that woman, don't have anything to do with that woman. If I alert her doctors when she seems to be behaving irrationally, I don't apologize for doing it. I reward and reinforce any positive behavior my mother displays. Everytime she reponds normally, such as writing a nice letter, I respond in kind. I am saner, she behaves better.
Two are needed to keep a war going. If you stop your internal battle, her flailings about with the JW stuff will just be a bunch of fluff you can wave off. For instance, as soon as she brings up the JW stuff, you may respond with a stock answer. "I am happy that you find satisfaction with the WTS, mom, but I promise you, I will never go back." If she persists, repeat your answer until she gets bored. Toss out anything she mails you. Delete her e-mails or block them. Stop responding until she starts behaving normally again.