I had B look over my letter, and he made some very helpful suggestions which I think makes the letter a lot more focused- for instance he suggested that I don't need to address everything she says in her letter, because while I can refute her specific points forever, that doesn't get MY points across. I think he was right, so I took most of that stuff out.
Anyways, I edited my previous post to reflect the letter I ended up actually sending to her. I don't expect a response, but I feel like I've finally let her know the way I really feel about how she has treated me since I told her I was not religious anymore (nearly 10 years ago), after building up resentment for years and years but never letting her know what I was thinking. And I feel really good about that.