Jourles,
My advice would be to ease off any discussion of Watchtower doctrine/policy and cultivate the family relationship you are still able to enjoy. Jehovah's Witnesses have a "defense mechanism" that immediately creates barriers to rational discussions. When you are arguing, they are only thinking of how they can "overcome objections." I doubt any of your sincere efforts are really making a difference in your mother's attitude.
It has been my experience that such discussions only help individuals who already have doubts about the Watchtower Society. Then, it becomes a valuable tool to sort out the falsehoods of the Watchtower and an aid to make a complete break from the organization.
But if you appeal to your mother's affection by being a good and caring son, she will have little defense against the greatest weapon you have: LOVE.