You have come a long way in your journey to self-discovery. You've grown tremendously and you should be proud of it. You are a much better person now (not that you weren't a good person before) to yourself and to your kids. Eventually, your H will see that. He is fighting your changes. He wants to be right. He wants to justify his reasons.
You are probably right that he is mourning the "loss" of OW. It was not by choice and he is angry. His "plans" have fallen through and he needs to cope with it. Unfortunately, you are getting the grunt of his "coping". Don't take it personally. It is between your H and his emotions.
So sorry that you had a break down yesterday. You are allowed to have a good cry. In fact, I think we all NEED a good cry from time to time. You've been through A LOT lately, and while you have done an incredible and admirable job keeping your head high and acted calm and mature, you have lots of built up emotions.
I'm sorry that your H is still talking about separation papers. Maybe he hasn't hit rock bottom yet, and the papers will "help" him get there. He doesn't fully understand and see the effects of the separation. He comes and goes as he pleases.
Don't let the papers steer you off your course. It's a distraction, a test to see if your changes are real and are to stay. Keep your eyes on the goal. Your H is noticing the changes, but he is not yet willing to trust that there are here to stay.
Oh, I know those shark eyes too well. The eyes really bring out the fact that the WAS is "possessed", if you will. Your H is fighting with his demons and there is nothing we can do about it right now. It is a fight he has to do on his own.
Keep focusing on yourself and your kids! You are an amazing woman and mother, and an inspiration to many!
Btw, congrats on your new job! I hope your first day is going well!!