LC This needs to come out. It is an enormously healthy sign and actually I am cheering here, much as I know it's distressing for you. What you write about the bargains you have made over the years doesn't surprise me in the least. It's all just hindsight though, because until recently, you wanted to believe he wasn't as he is. That you are seeing him with more clarity now is the first proper sign of detachment, trust me. This is why I've been urging you to write it all down, because I knew that once you started doing that, you would have a powerful realisation and would start to detach and heal.
I think you are right that he is still in love with the OW and as you know, I am pretty certain the relationship is ongoing. This is the only thing that's still worrying me about your beliefs, that you want to believe it's over between them.
This is going to be painful, but he was staying put because financially and domestically, it suited both of them. You mentioned months ago the amount of debt he was in and he therefore simply wouldn't have been able to finance a home for him, her and her DD and also meet his financial obligations to you and your son. I've got no doubt that if either of them had received a windfall, they would have been off like a shot.
It simply suited him to bide his time and the same goes for her. He didn't have to do very much to stay put did he? He was still sexually attracted to you for a time and it was not onerous for him to give you sex, but he even started to renege on that in the summer, didn't he? He had long stopped giving you any emotional support, so he managed to get a roof over his head and his basic needs tended to, without having to give very much in return.
The awful thing was that you knew none of this. You thought that this was your fault, because you were feeling so sad and depressed after the bereavements, your son's attack and bad news that kept coming. He let you think you had driven him away by your response to these awful events, when in fact his affair pre-dated any of them happening.
This is why timelining is so important, if anyone else is reading.
Let it all come out LC but you will not get any contrition or answers from him for a long time, if ever. And this must stop from your DS's point of view - although he agrees that his Dad has behaved monstrously, he shouldn't have to witness these scenes at his age.
Your best revenge now is to screw him financially and move on. Start getting your legal position sorted out now. Go to the counselling and now you've started, keep writing down everything that happened to you.