I am always facinated at the common line of thought that our parents use in conversations and letters to us.
IF YOU CAN'T STAND JEHOVAH'S WITNESSES THEN YOU PROBABLY WOULDN'T WANT TO COME SEE US, BUT IF YOU EVER SEE THE WORLD AS GRAMPS AND I DID PERHAPS YOU WILL WANT TO KNOW US
PERHAPS SINCE WE ARE JEHOVAH'S WITNESSES YOU MAY NOT ENJOY OUR COMPANY ANYMOREThis reasoning is very frustrating because they make the assumption that having a disagreement with the organization means that you no longer wish to have any contact with them.
It is difficult for them to fathom the thought that one can disagree on some things in life and yet still have a relationship. They fail to see that it is THEM and not you or your children that have cast them off.
Whether you agree on things or not fails to change that you are her son and her grandchildren are her grandchildren.
I think some sort of reply should be sent to your parents to help them appeciate these things. You've done a good job in explaining that it is actually HER that has made the CHOICE to have things this way and it is her that has become intolerant of the conscience of another.
Since she is your mother, hopefully you can treat her with the respect she is owed down to the end, with an understanding of the mind control she is under. At the same time you cannot make any compromises in your stand on issues you find conscientiously objectionable.
Emphasize that you are merely taking a break from the 'routine' until these important matters have been addressed and clarified. Ceasing meetings or field service hardly means you have "left the truth". (okay, so we are using a bit of theocratic war strategy.. lol)
Hopefully she can appreciate one day the high ideals of integrity and loyalty to something higher than an organization. I thought it was good that you showed that even the Society would find the content of her letters objectionable. (These letters are good to hold onto as they reveal the witness mindset that doesn't have a lawyer proof-reading their letters)
Revising your letter to take out anything that could be considered 'anti-Witness' might be something to think about. Highlighting that you are still a Witness and that you are merely trying to fulfill your Christian responsibilities to the best of your ability might be helpful. She has to agree that we are judged individually on this and not as an organization.
In the end, it is difficult to comprehend some Witness parents ever appreciating these points. The choices we make come at a price we knew would have to be paid. Your mother regrets the path you have chosen and so will cast you and your family off until you and your family are the people she wants you to be.
I hold little hope for my parents, and from what it looks like, your parents seem very similar. You just have to assure them of your love and them move on.