Vapo beat me to it but I was going to ask what rumors could possibly be worse than the truth. Also agree with walking away whenever she brings up OM, OM's wife, OM's dog, OM's car, etc.
I'm a firm believe in tough love when dealing with a wayward, I don't know of a single case where someone was able to "nice" one back. You should be furious about her disrespectful behavior, I don't know how you can sit there and listen and validate when you know what she's been up to. It takes a unique kind of person to bang an affair partner in a school full of kids. The moment I found that out she would have come home to all her junk sitting out on the lawn, and I don't mean neatly boxed up either.
Me: 59 w/ S17, D23, D26 Current R: 4 years Previous M: 21 years; BD: 06-14-12; S: 09-10-12; D final: 03-17-14; XW:56