Thinking fast today... dangerous, I know. So if we're at MC, and I happen to say that for M to work, A would have to be over, and you couldn't start any new ones, how could MC not ask? How could this not come out? How on earth does she not get confronted by this at MC. Does she try denial again? That wasn't a winning strategy last time.
Why do you always seem to be looking or a way to "trap" your wife into saying something or admitting something, or -- worse yet -- getting the MC to somehow get her to do it? GOOD GOD, you overthink everything!!! (YOU ARE JUST LIKE ME!!!!)
Why not just simply say "I don't really want a divorce, and I'm willing to work on the marriage, including my own issues that didn't meet your needs. However, I'm not willing to do that as long as there's a 3rd person in the marriage. So you can deny the last affair all you want and you can deny this new guy all you want, but I'm done talking about it and I'm done wasting my time and money here. This insults my intelligence and not only is it incredibly disrespectful to our marriage and our family, frankly -- it's very unattractive."
