Frankly, just don't leave the event, however boring and tedious it might become. You haven't been to your first MC session and it hasn't occurred to your WH yet that HE needs to think about such things. It's an illustration of several of your points above to bring up with your MC.

I wouldn't bother confronting her. It's been my experience such confrontations seldom accomplish anything and can too easily descend into unpleasantness for all concerned. She already knows she did wrong, and did it knowing WH was married. She has no sense of honor or integrity and you're not going to be able to instill any in her. That you're there with your husband and smiling in your happiness will send a far stronger message to OW than any words you could devise.

How To Organize A Marital Recovery Plan
Spying 101
Exposure 101"