All your responses seem to be about his responses, what he says, how he thinks, what he accuses you of, you're too scared to do this, say that, because of his reactions. Then justifying yourself constantly, reasoning and explaining.
He had an affair with a married woman - he cheated on you and she cheated on her husband. I don't give it shit if they kissed, didn't kiss, had a fumble in the car or a full blown shag - they both cheated and it went on for months - that you know about.
The only thing can possibly be acceptable to anyone in any circumstances, who decides to stay with a cheat, as you have, is that he cuts all contact with the person he cheated with. End of discussion. He still works with her, he's still friends with her, he still spends time alone with her
THAT IS COMPLETELY UNACCEPTABLE IN ANYONE'S BOOK, so why the fuck are you tolerating it?
It's a straight down the line 'her or me' situation and from the sounds of it, he's made his choice and it ain't you. He's going to carry on being her boss, her friend, he's going to carry on putting her over you, and he's going to carry on spending time alone with her, and you just have to suck it up if you want to hang on to this prize of a man. Prize arsehole more like!
Tell him to fuck off. Seriously! Get some dignity and tell him it's over, either he goes, or you go ... but please stop putting yourself through this torment.
It's almost unbearable to read to be honest. Why are some women willing to be such doormats?