It sounds like the session went better than you think. husband listened to some of it, likely a lot more than even he realizes. He also admitted that he can see the strain that staying up all night is taking on you. That is a LOT more than you thought he was seeing. in my opinion it is pretty huge.
Guys are strange about therapy. Years and years ago, when I was in elem school, my mom dragged my dad to a therapist. Then pulled my bro in for some sessions and then me in for 2 or 3. I wasn't in for as many because a lot of the problems were my dad and gfgbro's relationship - really super unhealthy. My dad resisted the therapy with every fiber of his being. My mom actually thought he didn't hear 99% of everything said there. I do mean everything, not just what the therapist said. This was in the 70's or very early 80s when NO ONE outside of California went to tdocs for anything. NO ONE knew they went - I didn't even know until the very end. Dad only went because Mom said it was that or divorce (she has used the d word no more than 3 times in their 40+ year marriage - NOT because things were great but because it was only for the most serious things for their relationship. Neither of them wanted us kids. NOt mostly a joke - neither could handle us alone.).
Just a few years ago my dad shocked us all by talking about things THEY learned from the therapist back then. He absorbed, remembered and USED most of what the therapist said. I do NOT think this is uncommon for guys. So if husband is resistant to going, don't worry too much. Keep going anyway.
Scheduling time together isn't fun or spontaneous, but it IS a good way to make sure that you have that time. Refuse to wear yourself out this week, esp on Fri and Sat. difficult child will pull all that she can to keep you from going. Unless it is likely to seriously maim her (sever a body part or produce more than 1/2 cup of blood) or someone else, ignore her shenanigans as much as possible. She will want to stop you from going to prove she is more important to you and because she does not want to share you and because her issues with you and ex getting back together.
Make the time you plan with husband SACRED - not just important or a priority. Deal with difficult child type stuff later. PLAN to be rested and well enough to go, then go if it is at all humanly possible. Same for nights you schedule to be up. Force difficult child to be in her room and quiet. Ignore what she does, do NOT respond to her if she yells, if she comes out then go away from her behind your bedroom door. If she pound son it and yells, open the door and throw water in her face and shut it again. Make sure she knows that you WILL address this stuff later, but at the time act like she isn't there.
If water in her face isn't the right way to handle it, do whatever will work. Make SURE that she knows that YOU WILL MESS UP HER TIME TO ENJOY SOMETHING ALSO. So far she plays the yell and scream and disrupt mom and husband game and is sure that you may "punish" her but it won't be a big deal. When she is watching a favorite movie or show - pull the **** she pulls. Let her know you are doing it because that is what she does to you. Afterwards, when you are BOTH calm, talk about it. Tell her that you did it to let her know how awful it feels to have someone do it to you.
A lot of the time your difficult child does stuff and no one does it to her. You are so thrilled when she does something on her own like watch a show, or when she does something that isn't pestering for your attention every second so you try do you your thing when she is occupied. She doesn't get a taste of what she dishes out. At least not from you.
I am not saying to do this all the time. Do it a time or a couple of times so that she sees exactly how awful it is to have it done to her, how "unfair" it is for someone to do that. It likely won't cure her, but it might help her learn to stop and that you WILL be creative enough to make her regret doing it to you.
I don't know if that will work, but what you are doing now isn't working with her so what do you have to lose?
Anyway, do everything you can to make this date with husband a priority. Even brainstorm to find dates that are cheap or free. Heck, maybe you and he can start going for a walk, just the 2 of you, for some of the time you have together on his day off. With or with-o the dog(s).
I hope that you can figure a way to work things out. He is right about difficult child being overwhelming. And he doesn't have a board to vent to. He only has employees. As for his ex, could he put a block on texts from her and calls from her during date and family time? No reason she needs 24/7 access to him. He doesn't need to tell her, just let voicemail pick her up and ignore texts except at specific times during the day or week. Not calls from the kids, calls from her. He doesn't need her fb, nor does she need his, in my opinion. Can he unfriend her and set privacy so she cannot see him and he cannot see her? I thought that was one of the "great" things about fb?
These are just ideas. He may have to ease into these things. He does have a LOT to remember and a lot of years of telling ex just about everything to unlearn. I know you hate her, and when he talks to her, but 4 years really is not a long time to unlearn those patterns.