I'd write back. Something kind of like this:
Dear Mom,
You will be thrilled to know that I have not turned my back on Jehovah. I have merely left a religious organization, which is not the same. I know you do not understand that. If you would like to have a discussion about why I have doubts that the WTS is Jehovah's mouth piece, I will go over those concerns with you. Perhaps you can help me by clearly answering my questions, one topic at a time. Until that happens, I am going to have to do what I feel is best for me and my family.
Thanks for the Bible. Could you please show me a verse that speaks of "paradise earth"? In those EXACT words? I know that it has been preached in the WTS, but I cannot find "paradise earth" anywhere in the Bible you gave me. If such a place exists, and we are to be preaching about it, surely "paradise earth" is mentioned! I can only find paradise as referring to a heavenly place. Perhaps that is an area you can help me with. "Paradise earth", in those exact words, within the pages of the Bible. Please tell me where I can find it.
Did (name six children here) return to the JWs? I wasn't aware of that! How happy you must be. I'm the only one left who hasn't. That is too bad, but like I said, until my questions are answered very clearly, I cannot.
Mom, please take another look at the picture you sent me. Do I really look happy there? Perhaps you didn't see it, but I was not a very happy child. (Name some of the reasons, without accusing her.)
Could you please explain how I am immoral and selfish? What do you know about the life I live now that would lead you to believe such a thing?
You also called me a worthless sinner in your letter. Aren't we all? Don't we all fall short? Who do you know within the KH walls that does not?
I do love you, Mom. Even though you referred to me as "something awful" that you gave birth to. When you read the Bible, do you find that Jesus said or encouraged others to say such things? Where does Jehovah instruct mothers to say such cruel things to their children?
In spite of the hurtful things you said to me, I do still love to hear from you and hope you are doing well. I do still love Jehovah and I do still read the Bible. I will no longer read WTS materials, until it is proven to my satisfaction that they truly are God's organization. If you (**and don't let her try pawn you off on someone else) are willing to go over the things that I have concerns about, that would be very helpful.
Love, nj
Maybe she would converse with you, even in letters, and you could give her some eye openers with the questions you ask. But keep your cool and keep her on one topic at a time! This has been working (slowly, but surely) for me. By making her "teach you", she will be teaching herself that the WTS is not what they appear.
Best of luck!