Excellent letter. But only if she'll read it.
I agree with Serendipity: The Bubble reference is a conversation stopper. Also, why not appeal to team spirit by re-working the following:
The idea that this is somehow “working against Jehovah” is ridiculous. And, dear, you can’t win if you believe salvation comes from following man.
Perhaps something more like: The idea that acting as a responsible, mature Christian (Eph. 4:11-15) is somehow "working against Jehovah" is contrary to the Scriptures. And, Mom, we can't be pleasing to God if we believe that salvation comes from following a group of men, rather than our Lord Christ Jesus who alone is "the way, the truth, and the life" through whom we may "come to the Father" (John 14:6).