I echo the comments about trusting your gut. I suppressed mine for 2 + years. I really wished I had not.
I caution you about continuing MC. Read Not Just Friends. My SAWH, upon confrontating him with the name of the woman he was seeing, insisted they were just friends. And he did this in MC, too. He continued to do this for 2 weeks until I found more evidence that this had been going on for MUCH longer than I suspected.Even then, it has taken MONTHS to get answers to everything...and I am sure there is more to know. At the very least, your H's boundaries are for shit in that he is spending time with the person in question instead of working on the marriage. At the most, he's having an affair. Neither scenario is acceptable.
Time to 180. You can read about it in one of the posts in the healing library and I have also seen it before in a few of the posts for Newbies on here. The main purpose is to help YOU but sometimes a side benefit is that the WS takes notice that you aren't consumed with what they are doing like you might have been recently.
Good luck. Try to stay calm and centered and rational. Does the OW have a spouse? I might check in with that person at some point. IMO she is a conniving bitch who has put herself into a really bad position. Women are terrible to one another, aren't they? I would never, ever do that to someone, but I guess I am more secure than OW are. Funny how we find them so threatening. In reality, they are probably more threatened by us.