This is a difficult situation for sure. The only thing I can say is that your goal to hold your step-son accountable for his actions is the right one. If you feel you only have two years left, it would certainly be nice if your son could learn how to conduct himself. Maybe rethink your boundaries and share them with him. Don't give in, it won't teach him a thing. I wish I would have known that years ago.
It's obvious by your writing that your son has been enabled. Don't beat yourself over that. There are many of us who have traveled this road. Maybe ask yourself, is this going to help my son move forward in life or enable him to continue his destructive conduct? I can't imagine your suffering and still trying to care for your husband. Plain and simple, if your son is a caring individual who appreciates all you did for him, he would want to be part of the solution for the two of you, not the cause of such stress. If you need to go no contact for you, then do it.
As for the dog, I understand totally. My daughter has two little ones. If we could get her to go to rehab (she is 40), the only option is the same as it is for you. I have done the calling, to no avail. We don't live in the same state. We are not in a position to take on the dogs. It's so hard. I wish I had answers for you. What would he do if he didn't have you to dump his dog on?
Please take care of yourself.