You go to all the trouble of spending years adjusting your attitude to accommodate all of your in-laws' quirks and irritating behaviors, and then discover that if your M is dissolving, they will take sides against you.
Yes! They are a very high-maintenance FIL and MIL. They will undoubtedly take his side, and that's okay. As long as they have the whole picture, they can view it through whatever lenses they like. The main point of my letter would be to express my sorrow in the end of the relationship(s), with just a mention of H's choice to reconnect with OW. I don't expect it to accomplish anything, except perhaps to help me to move on.
M60 H52 D20 M14 yrs OW-old gf from 1986 bomb-5/18/08 H filed for D-9/10/08 D final 4/24/09 xH remarried (not OW) 2012