I think this could be a "turning point". I think the whole divorce talk really slapped in the face. He probably did not mention OW because he thought you were/are on the verge of a divorce and mentioning the OW would push you to that.
IF you want your marriage to work, I think he needs to know that you know about the OW and will stick with him if he leaves her and have the wonderful marriage y'all both want.
When I read your situation, it just seems like a crazy cycle. Something has to break that cylcle IMO.
