Quote: It's just that I see that my xH is so deeply entrenched in this R with her now that I can't imagine him getting out of it anytime soon
Entrenchment is what you want because that's the only way they are both going to have to deal with the reality of the situation.
Honestly, the more I hear about your husband, the more I think you could be doing sooooo much better.... It sounds to me he's living with her because he needs someone to take care of him financially. If that's so, eventually she's going to resent that.
Quote: I've dipped my toes in the dating world a little and I'm just not ready, I've realized. I am far too injured to be putting myself out there. So, when it comes to GAL, I guess I'm going to stick to platonic friendships, solo ventures, and things with the kids.
That's completely normal. It takes at least one to two years post-divorce before most people are even ready to start dating. You have a lot of baggage right now that you're going to need to work through. In the meantime do have platonic relationships, both female and male. Of course you don't feel happy right now, but it will happen. It's just going to take time.
Treat yourself gently and do good things for you and your son now.
One more thing... with your son. If you have any sense that he may not be safe or well take care of (there may be a lot of tension in the house where H and OW are), you may want to go ahead and pick him up when he's upset or not feeling well.
From what you described, that situation did not sound good to me....
There is no arriving, ever. It is all a continual becoming.