Your situation is so tragic, but not unlike so many others here. It is hard to understand a woman that does not want to try and preserve such a beautiful family.
I dont think you should send that letter to her father.
I think I understand why you want to. You probably believe that her father has a larger influence on his daughter than you and you are hoping that he may talk some sense into her. It may be that his talking to her could buy some time, but I think she will be so furious that you gave all these details to her father.
Also, consider that what you are saying in this letter is going to cause her father to lose more respect and love for his daughter. You dont want this. Take the higher ground.
Believe me, I made the mistake. I was panicked but sincere. However, I did not know of the fragile nature of the relationship between my W and her mother. I now realize that their relationship would have been seriously damaged anyway if a divorce occurs. I just have regret that I was the one to have been the one to notify my MIL that our marriage was in crisis.
