I am so sorry you are going through this. My heart just aches for you.
I was reminded of something I read about the other women in general. You mentioned how she reaches out and snags him back after he tries to end it. What I read was that the majority of other women starts giving up after about two or two and a half years.
I imagine she's fought with him when he gave her excuses about staying with you. Maybe he's seen her when the mask slipped out of frustration. Maybe he has second thoughts about her wonderfulness.
It almost sounds that you're certain he won't leave you for her.
That's a good point I guess, but he needs to go all the way and go NC. Period. The longer he puts this off the more likely they become one of those "make up to break up" couples.
The other thing is to blow it up and tell the OBS. Make sure you have your proof because I think you mentioned OW thinks you are the crazy wife.
This will give you another pair of eyes on them and will give you some breathing room as she scrambles to play nice with her H.
he has a male friend he confides in and has even said he’s trying to end it for good (I saw when I checked his messages a while back before our most recent discussion ).
I'm not sure if this male friend told you this himself or if you read this as a text to your H.
If it was a text, I'm not sure it means very much. As a text, it's something I would say to an alcoholic friend. "Yeah SuzyQ, I know you're trying to quit. You're doing great!"
It means something else when his friend says to you that he knows your H is trying to end it.
By the way, are you close to some of your H's male friends and can get them on your side? I also love how you take pains to blow up their little plans. I did the same things. It brings out the worst in predatory women.
I hope this thing resolves quickly for you for your own health. Your most important tools are NC for your H, demand it now, and give the OBS the gift of clarity to make his own decisions.