Hey sweet friend. I'm glad to know that things are going well in your reconciliation. You are the real DB champ! I know you guys can make it! Keep up the good work. We are all pulling for you!
One thought regarding the letter to OW...it might be better that he just end all contact with her period...no letter, no more texts, emails, phone calls, etc. If she's starting to back off, a letter may stir her up again. Some crazies see ANY contact (even if it's negative in nature) as a sign of interest. Just something the two of you may consider and/or discuss with a MC before he sends it. It may be very freeing, though, for him to write the letter and then just destroy it.
I'm looking forward to hearing about Retrovaille! I hope it's a great experience for the two of you!