Your biggest obstacle will be that the children live with their mother and she has more time to fill their heads with her spew of venomous sh**. You will lose this battle unless the children can be won now. And that will entail him having custody of the kids. He has a case with the courts, with the fact that she was sleeping around. However, they will slam him with mental stability issues so it will be a battle.
This hits very close to home because my mother did this same thing. She slept around and then when my dad go tired of it and left he was disfellowshipped as soon as he got a girlfriend she pulled him before the elders. Though her infidelity was never mentioned because my dad just wanted out. From there she filled our heads with trash about how horrible he was, about how he left us because he didn't love us anymore. About how he was sleeping around and what a horrible person he really was, how he beat her and would have us too if she hadn't sent him away (no her stories never jived but as a kid who notices?) We tried to see him, but we would get a silent treatment when we got home. We tried to talk on the phone with him, but she always had chores that we had to do RIGHT then. The summer before kindergarten my sister and I went to see him and when we came home our pet goat was gone. My mother said that he strangled himself in some string because we weren't home to play with him. It was our fault the goat was dead, then she didn't talk to us for two weeks. When my oldest sister turned 16 my dad bought her a car so that she could drive us to see him. My mother chose this as her opportunity to tell her that my dad wasn't her dad. She was the product of mom's first marriage and yet she had only known my dad as her father since they married when she was only a year old. She made a "deal" with him that if he'd quit calling and coming to see us then he didn't have to pay child support. She then told us that it was his idea. That he didn't want anything to do with us and so he wasn't going to help support us anymore. A few years later she turned him into the DA's office for not paying child support and had him thrown in jail. She made it so difficult to have a relationship with him that we quit trying.
It wasn't until I was long out of the house and out of the Borg that I was able to sit down and talk to him and realize that everything that he had to say was true. He was a genuinely great guy who didn't want his children to be separated (he never tried to get custody of us because he knew since the oldest wasn't his they would never give him custody) so he didn't try. God how I wish he would have now!
Anyway, you are up for a long battle and unless you can get through to those kids and keep in their lives so that they know for themselves that you are both good people. You need to work extra diligently to keep the full time parent from filling their heads with trash!
I am not sure what you mean about a cease and desist letter, but if you think that calling the woman on the table is going to help, think again. She'll just turn it on you and she now has it in writing. The JWs are masters at reading into written words, just look at the philosophies on the bible!
May you be successful in getting those children to know YOU and not know you by the Borg standard.
Good luck!
Good luck to you~