BW05, Have you ever heard of gas lighting? If not, do an internet search on the subject. Do, I found his reaction odd? Not at all....this is what gas lighting is about as well as projecting on to you what he's done or his doing or even thinking of doing. It's very typical behavior for a person in an affair to do this. Trust your gut.
As for outing the affair, well...you could do this, but what is your main reason for doing so? To get them to break up and hopefully he'll suddenly wake up and run back to you? If that is a yes, then you need to rethink outing the affair. Yes, many times the affairs are outed, but what happens is that when you do this, it will drive him closer to the ow. In his mind, he needs to protect her from everything that you are saying. He needs to rescue her and keep her safe. He has to come off looking like the good guy and guess what...your actions become even more justification for why he's doing what he's doing.
Allow the affair to die a slow and natural death. The less you talk to him about her, the better off you are. He's irrational right now because he's higher than a kite just knowing she's there waiting for him. Don't give him or her the satisfaction of knowing that they are taking up space in your head.
She's truly nothing more than a crutch while he's figuring himself out. She's nothing special and the only thing she's got going for her right now is that she is "new". Once the newness wears off, she will begin to tarnish and all of those things he loves about her right now, will eventually turn him against her, i.e., as he slowly begins to return to earth.
Keep the focus on you and your mantra is "you are the prize". Repeat that each and every day because truly you are the prize.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.