I'm sorry for what you are going through. It's really tough.
Here's my general take: DR does talk about boundaries: the Last Resort Technique, After the Last Resort Technique, and even ultimatums. Generally, these boards tend to not to discuss these things. If your husband is in an open affair and won't stop, and still lives with you, it might be a good idea to explore those options.
It's natural to want to "nice" a cheating, unrepentant spouse back into a marriage. Kill em with kindness, right? Wrong. Unfortunately, it generally doesn't work. This affair really isn't about you, it's about him. The longer you try to nice him back, the more you will personally disintegrate (emotionally/physically/professionally). In addition, the less he will respect you and desire you. What he's doing right now is called cake-eating.
I allowed my wife to do that while she was having an affair with my best friend. Needless to say, I'm divorced now. My wife, in the end, couldn't respect someone who made their own needs so small, and became a milquetoast and happy cuckold. People are attracted to strength, centered-ness and clarity. You are ANYTHING BUT clear, centered and strong when you are being torn to shreds and you are trying to people-please and walk on eggshells around a cheating spouse.
Here's my general advice in answer to your questions:
1. Get that PI to get proof for your lawyer. Prepare yourself, and your son, for worst. Prepare yourself for a life without him, with you keeping most of the money and the house for yourself and your son. Think of it as empowerment. You are doing what is necessary. You don't have to act "as if" and pretend you're groovy with him having an affair. Don't become the woman he can't resist. Become the woman YOU can live with. Take care of you for YOU. THAT woman, who takes care of herself, and takes care of business, is infinitely more attractive than a doormat.
2. You don't need to respond to his affection if YOU DON'T WANT TO. My opinion is cut him off. He needs to feel consequences for his actions. He can't have you both. Let him choose.
3. Ask him to move out of your bedroom. Tell him you are not comfortable sleeping in the same bed while he's having an affair. IF HE has the affair, he needs to move the other bedroom. Period. YOU shouldn't unseat yourself as the woman of the house and skulk off into another room because he's having an affair. He needs to pay for his actions. Make sense. The ring issue is purely up to you. WHAT DO YOU WANT? You can't use him a gauge for all your actions. He'll smell your desperation and exploit it. If you want them off take them off. Period. The more you act authentically, the more power to take back. Stop acting like your a footnote in his life.
4. Do not chase or pursue him. That has the reverse effect you intend. It smothers him. Also it lets him know you are a viable plan "B". Why should he decide between you when he has two women vying for his affection?
5. Start to set some boundaries. Financial? Spending your income on his affair partner? Can he text/call her in front of you? If you enable his affair, you aren't helping him. People respond to consequences for their actions. Really, they do. They also make you a more formidable, substantial person (which is, believe it or not, more attractive than a doormat). You are doing him a dis-service by allowing him to behave cruelly, immorally and selfishly without consequences.
You can't be his best buddy and listen to him talk about the OW. That means you don't value yourself. He won't value you either.
Take a look at the last resort technique in the Divorce Recovery book. It might be the first bit of sanity you feel. Then, try the AFTER the last resort technique (going dark). Then, the ultimatum. (Have all your legal ducks in row before giving him an ultimatum). Try the Last Resort Technique for 2 weeks, then the After the last Resort Technique for 2 weeks, then an ultimatum. That's if you can last another month.
Or you can simply say to yourself, 2 months of tolerating his affair has been enough. Get the PI evidence ASAP, and then give him an ultimatum. Don't tell him what to do, simply say, "I don't want to live in an open marriage, this is intolerable to me. IF you don't end it with OW and recommit to our marriage it looks like I have some decisions to make." Then walk away. Approach him the next day and ask if he's made a decision. If he says he's leaving her, then he must write her a goodbye letter and show it to you and initiate NO contact with her. There's good material on this on the web. If he hasn't made a decision or says he wants her, then divorce his a$$.
5. Take care of yourself. Exercise, take up some hobby that takes your mind off the affair (hiking, karate, rock-climbing). Find your inner warrior again. Become ferociously strong and beautiful.
My personal opinion is that infidelity is a pattern for him. He's morally weak. Given your age difference, I think he'd be worshiping the ground you walk on. At his age, he's supposed to be showing wisdom and strength. Instead his being foolish and selfish.
It wouldn't hurt for you to get a little mad (it often gives us strength) and realize you can probably do better. Get your game back. Stop being thrown off balance by his antics. Start making him worry about what his life will look like when he loses you.
