I would appreciate it if you would contact Laurie for me. As for the letter to in-laws. What kind of R did you have with them and do they seem mad at you because of the separation?
If you had a good R, then I don't think it would hurt to tell them that you don't want a divorce from their S. However, make sure you frame it in as positive language as possible. Like instead of I don't hate you, maybe I still care deeply for you both and hope we can remain in contact. Just reread it and take out any negative sentences.
Everything happens for a reason, maybe Dad needs to find that it isn't better out there, he needs to realize how good he had it here. Maybe he will find God and that is the most important thing when he finds Him he will know he is supposed to come home.