It's reasonable to be nervous about taking action - once you do anything you can't undo - but this situation was/is not sustainable. You are courageous and absolutely doing the right thing.
Don't second guess yourself. Kids don't turn out 80% ok because their dad was ok 80% of the time. That's just not how it works. A parent can absolutely do enough damage on bad days to make the good days irrelevant. The bad days, however infrequent, still paint a disturbed picture in your children's world - still set them at risk for negative relationships, drug abuse, depression, etc. later on. The sooner you can get them away from him, the sooner they can begin to heal.
I disagree with your therapist - it doesn't sound to me like H is "trying" at all - it sounds like he's shifting blame onto the kids, and taking no personal responsibility. Immature at best, narcissistic at worst. At any rate, you can't work on behavior if you think it's ok or non-existent. I, too, question her judgment.
As for custody, he may or may not even go after it (perhaps for appearances, but then realize he can't handle them on his own for any amount of time). I know it's a scary idea for him to have custody, but getting it doesn't mean he will be able (or want) to keep it, either. Something to keep in mind, anyway...definitely document everything (including your conversations with him about it). Huge hugs!!!