At the end of the night was when she came back downstairs to tell me how angry and abused she felt by me not working with her on the divorce. Especially custody, she thinks I'm refusing to work with her at all on that, that I'm leaving it up to her. I might have told her to come up with a plan by herself and I would review it with her. That just reinforced to her that I'm unwilling to do anything as a team with her. In typical fashion, I was at a loss for words at the end of the conversation, so she had the last word, telling me to write her a letter (since she was done waiting for me to talk
She was not asking for a letter of apology!
Look, I well understand the frustration on her end. My H would shut down and say nothing. The more he shut down, the more upset I became. It was a vicious cycle. I could not understand him and how or why he couldn't just open his darn mouth and say something. I would get so hurt and angry, I would say terrible things trying to get some kind of a response from him.
Based on your description of the conversation, she wants you to write down what you want in the divorce, custody, etc. You were flooded and couldn't find the words to speak, so she's telling you to write them down.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!