Your response is a help and contains a good idea. You remind me of my initial approach after the separation. Keep to the plan. Say only what needs to be said (emailed). I've developed a good life with my son. I have legal custody. I got there through a legal agreement, not a fight. She was in a series of worse and worse jobs and found a cheap apartment in a town with terrible schools. With my son staying with me, he got to stay in the good school in my neighborhood. Travel to his mom for visits is an easy 25 min away.
Your support is great at a time when I felt, "Oh man, she is trying to pull me in." I would not be pulled in though I did find myself subject to the emotional shrapnel thrown off by the black hole she lives next to.
What I've got to hear and do is - Stay Strong. And more important - Be Sure to Enjoy all that's good - with my son.