Free, I'm trying to speed read your situation, but I see a ton of parallels.
First, the only thing that changed the dynamic was once I found out about the OM. I spent about a month in hell gathering intel and working through my emotions. I needed that long to figure out if I would stay with her if she broke of the A and to decide if I should castrate the OM. The answer was not yes to both (although I'm rethinking both as time goes by). I also needed the time to figure out specifics of what was going on, and to take pics for potential future use. And to talk to safe people. In my case I don't have many, because I knew if I disclosed anything to my family they would never let her back in and they would constantly pressure me to dump her.
Once I felt more centered and confident I knew exactly what was happening, I confronted her. I told her that I had some questions, and that I knew the answers to most of them. She should not worry about what I knew or how I knew it. She should simply tell the truth. Because I KNEW.
The important thing to me is that I did not "Blow up" the marriage. I knew I wanted to give her a choice to do the right thing, but I didn't want to pressure her into anything. I wasn't even sure I wanted to continue, or file for a divorce. I simply knew that I would be OK if either happened.
TO me, you still are operating from anger. Anger is totally normal, but you don't want to make huge, life-changing decisions based on emotions. Try to step back and evaluate your other emotions. There is no turning back from your choice, so please, please make sure it is the right choice for you. I can't explain it, but there was a moment when I knew what I had to do. And a moment when I knew how I felt about the situation. They were separate and painful conclusions to reach, but I can guarantee that I would not still be married if I took the approach you suggest.
You may justifiably ask if it's a good thing to stay married after all that's happened. That is the question you should be asking yourself, not how to approach the confrontation.
Just my thoughts. I don't think my choice is best for everyone (far from it). I simply want to make sure you've taken enough time to really investigate what YOU want to happen.
Me-38 W-38; T-15 M-12; S10 and S6 BD - July 2016; EA confirmed; confronted Aug 2015 EA dissolved 12/2016
Never make someone a priority when all you are to them is an option - Maya Angelou