Frog: You've got a lot of good advice here and I'm sure you'll pick thru it and use what resonates with you personally, so I have nothing else to add.
Except... I detect throughout your story that your mother loves you and the thought of you being wrongly treated seems to upset her. It's this close family bond that causes many people to come INTO the "truth" (or stay) and the same emotional pull has caused more than one parent to examine their JW beliefs after a child leaves, especially if the child appears to the parent to be happy, well adjusted, and balanced in his or her new role as a "worldly person."
So I'd just say that, along with the doctrinal points you choose to use, don't forget the power of love. It's your best tool. Whatever else you "show" your mom from the written page, make sure you also point out to her that the dubs really don't practice true love, despite their claim that love is their identifying mark. I am certain you mother knows of many examples, as do you, which you ought to work into your conversation whenever the opportunty arises.