On first blush, it might look like your WH is trying to decide between his devoted wife and his long-term AP, but I think maybe it's you who's in the catbird seat, Coria. If you've ever read posts in an OW forum (and yes, there's a forum for everything these days, no matter how skeezy
), you'd see pretty quickly what the OW themselves can't admit... the married cheater doesn't WANT to leave home. He tells the OW that he can't leave because of the kids, or that his BW will be suicidal, or that his great-great-uncle Bertrand won't leave him a big inheritance, or whatever it takes to get her to stop pressuring him for more, but with rare exception, he doesn't leave his wife.
Your WH has had six years to leave. He didn't. The OW is pissy about that, so she figures if she stirs the pot, you'll toss him. And frankly, if that's your choice, no one could blame you. The guy cheated for more than half your marriage. He doesn't deserve an honest, caring wife.
That said, unless I was certain I wanted a divorce, I think I'd work on busting up the affair first and then make a final decision afterward. So, you set down your boundaries, meaning what does he need to do to keep you from bolting right now. Most people list out things like NO CONTACT with the AP. That's imperative. Usually, the two of you would create an NC letter which can be emailed or sent by snail mail, never in person. "Closure" is something people provide to themselves, so it's not necessary for them to meet. You'll probably want access to his phone, apps, and emails. You might even put a gps tracker on his phone, or put him on a flip phone, or whatever makes you feel like he's making an effort to be transparent. Just remember that a determined cheater will find a way. They can buy burner phones, turn off their gps, leave their phone in a parked car, whatever. The demand for "transparency" is mostly about observing his reaction to it.
Boundaries aren't really "rules" for the cheater to follow, although it kind of sounds that way. We can't control other people. But we CAN say what we'll tolerate and what we won't. So, your boundary is more like "I won't tolerate a secretive man who password protects his devices". Does that make sense to you? You can test your "rules" by rewriting them into boundary statements. That way, you're serving yourself, not trying to impose controls over a grown ass man who should have known better. The last thing you want is to get stuffed into the authoritarian "mom" role. It begs recalcitrance like a teenage boy sneaking out his bedroom window. You're not his mom and you're not his jailer. You're just telling him how a guy needs to behave in order to have a shot at a relationship with you.
Anyway, it looks like you've got options as to how you want to deal with your situation. Like I said earlier, no one could fault you for just being done with this cheater. If you're wanting to test the waters of R though, I think after a 6-year LTA, he's a risk for breaking contact. I think I'd download the divorce papers, fill them out, sit him down and show them to him, and let him know that the MINUTE he breaks NC, they're getting filed. Interview some attorneys too, so you can put one on retainer quickly if needed.
Cheaters don't take us seriously until we're actually serious. So say what you mean and mean what you say.
((hugs))