I wouldn't communicate anything to her in writing that is not related to the care and well being of S2. She has already shown a willingness to mischaractarize anything/everything you say or do to make you look evil.
I understand that you don't want a divorce, but that is a very real possiblity. With the custody issue yet to be decided, you do not want to give her anything at all that could be misconstrued, taken out of context or otherwise manipulated to besmirch (sp?) your character in court. Custody negotiations can be really nasty because there is so much at stake.
The hard part of all of this is dealing with one's own emotional pain without becoming vulnerable to attack. Write all the letters you want; they will help you deal with the pain. Just don't send them. There may come a time to communicate some of these feelings--after the dust has settled.
