I think you've done an excellent job.
It doesn't even MATTER "whose fault" anything is. It certainly isn't yours. I don't even really think it's his. It wouldn't be his fault if he got malaria while he was deployed, would it? This is kind of like that, really.
As far as the "therapy" he's been getting, if he's only been going in once a month, I'd be willing to bet all they're doing is asking "How's it going?" To which he's replying "Fine." (My personal favorite response to that question!) They might talk about his meds, then they send him on his way & send in the next one. They are ticking things off a list, more than likely, not treating people.
As far as "one session" goes, who knows? When my friend killed himself last spring, after talking some of it through with my therapist, he said, "Now I want you to hear a GOOD story." He told me about a Vietnam era vet who had been struggling with PTSD for years. Finally, one day, in the middle of a panic attack, he decided to go the the VA. It was about an hour drive. He made it a couple of miles then went in the ditch. (It's hard to drive when you're having that kind of panic attack.) A farmer saw him & went to pull him out. When he realized the guy was in no shape to drive, and the guy said that he was a vet, headed for the VA, the farmer gave him a ride. (We're talking about an hour drive here!) When they got to the VA, the guy couldn't make himself go inside and he'd decided he wouldn't talk to a Dr. They sent a nurse out to the parking lot to talk to him. She noticed that the guy was over weight and started talking about that with him. (He's still having a panic attack.) She asked if he'd be willing to go to a near by town to talk to a guy about his weight problem. He said that he'd be willing to do that, so they gave him a ride to see the guy who is now my T. And he's the vet's therapist too, and has been for a long time. The vet is doing much better. (Now he only sits in the garage with his gun watching for invading neighbors about once a week instead of every night.) The thing is, at least SOME of these people are REALLY good at what they do. I hope your husband crosses paths with someone like that, all the stars align, and there's a happy ending.
Regardless of that, I think you've done well and I've think you've done what you can do, Keep taking care of yourself!