I expect I'll get a roasting on here but I have been 'the other woman'. Here's my perspective:
He chased me, I wasn't interested. He chased me and chased me until when I was on the rebound from another relationship he told me what I wanted and needed to hear.
I reminded him that he was married but he told me that he was splitting from his wife and that's why their house was on the market (it most definitely was). When I split from my ex-h we shared our house until it was sold, sleeping in separate rooms so I could see that this was plausible.
He introduced me to his friends, workmates and other mutual colleagues telling them, in front of me, that we were meant to be together (Gawd it makes me cringe reading that now) so he was lying in plain sight so to speak.
Our affair lasted about 4/5 months. A friend tipped me off that he had finally sold his house but had bought another, with his wife.
I confronted him and boy did he squirm when touched with the truth stick. He was ditched quickly and I made it quite clear exactly what I thought of him.
About a year later I bumped into him and he was under the illusion that we could still be on friendly terms. The look I gave him left him in no doubt as to what I still thought of him and he actually appeared hurt and incredulous that this was the case.
Moral of this sad tale is that some folk are just liars and convince themselves that what they do is ok, like a game, like it's justified. Looking back, I can see this toad for what he was. I think from your post that your ex will have been economical with the truth and it is unlikely you will ever find out. Don't contact the other woman, what would the point be. One day you will be glad you didn't when you have moved on. Right now you are hurting but this will pass. If it was a bolt from the blue it's because your ex is as good a liar as the chap I was involved with. It's a game to them. Rise above it and move on.