Bright, I'm so sorry for what you've learned. Mlcers can turn their feelings off like a light switch. Their empathy chip is broken. I've been right where you are, i.e., questioning how can the love just disappear and I've given many years of thought to this question. I do think that they love us the best way that they can, but the love isn't as deep as the love we had for them because they are afraid of abandonment and being hurt by those who love them. Again, I think this stems from their childhoods.
As for the ow, that took a lot of nerve to follow you into the Ladies' Room and talk to you. Evidently she thinks that everyone should be friends and maybe that's all they have is a powsy wowsy friendship and not the type of "affair" relationship. It's very crazy if you ask me. She definitely is a predator and has her hooks into him and is influencing him in many ways.
I do agree w/Heather, it's time to set some boundaries and I would be getting my ducks in a row about the business you are handling w/your h, as well as considering a different vacation home. I'm sure you can locate other places to stay in the future and not stay at that vacation home. I'm sorry that you had to learn all of this...but in a way, it's good that you know now and not when he drops the paperwork on you.
I also think that all of the discussion w/friends about the divorce may be his way of letting you know what he's planning to do. They can be cowards and may tell friends so that those friends will tell you. Maybe he's hoping you'll file...I wouldn't. I would make him do the work...but in the meantime, I would be seeking some legal advice. Knowledge is power.
Please take care of yourself.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.