I agree with Puppy. Skip the letter. To tell you truth, as I read the letter, it seems a bit too much about him and her (although you'd probably be more comfortable there anyway etc...). By writing stuff like that, you are still giving him power because you are A)letting him know you are bothered by it and B) you feel he would be happier there. Both make you less powerful.
Additionally, you are writing a lot of stuff to him that is educating him as to the law. Why tip your hand like that. Now he knows what he has to fight against. You would be giving away your head start.
Do what you need to do. Do what you feel needs to happen to make things square. Take the $1000, get your paperwork in order. Let him pay $150/hour to learn how to deal with what is coming. Don't you give them info for free.
This is one of those therapy letters. You got it out, you put it on paper. Now burn it and release the bad energy. You are moving on.
Me 44 She 46 S13 D9 M18 T23 3 years DB'ing Successfully busted