I managed to get out of the house before WAS awoke. However, this morning I received a telephone call from an angry mother-in-law who says she spoke with OM (he is mother-in-laws personal trainer as well) who denies any PA (and presumably EA). She also says she knows her daughter and she would never do such a thing.
This is a cheeseless tunnel. My mother-in-law steadfastly maintained her daughter's innocence ("She says she's just a friend!") throughout, despite all of the evidence I had. I just continued to tell her and my FIL "She's lying to you. She's lying to all of us, I'm sorry. I wouldn't have told you this if I didn't have proof."
She's going to continue to hit you with all kinds of bile today. When she says you shouldn't have said anything to her mother, or continues with any of the "slander" stuff, just say "I'm sorry you feel that way. Everything I've done, I've done to try to save our marriage." You may want to also add "Since you brought up slander, I guess this is a legal issue now, and best left up to the attorneys."