Welcome Rayzorfish.
I really feel for you. Honestly I do. If you haven't grown up around alcoholic parents with all the toxicity that comes with them, it must seem like these people are aliens based on their behavior.
Unfortunately, your in-laws' behavior is pretty common amongst alcoholic parents, and not uncommonly can be worse when they stop drinking (that's where the "zero tolerance" for drinking comes in - because they could not control their own drinking, they can not believe that anyone else could possibly just have one beer or keep beer in the house without drinking it).
More unfortunately, Peter is exactly right. You didn't cause it, you can't control it and you can't cure it.
So what can you do? Be absolutely supportive of your husband in his counseling. Print out the sticky at the top of the 13 common characteristics of adult children of alcoholics. Ask him if he would be willing to take it to show his counselor. It will help you to understand where your husband is coming from and help him understand why he is the way he is. Talk to him about boundaries - setting them, enforcing them - both for you and for him. Talk to him about always having an "escape plan" so that any time you or he have to deal with his parents, you always have a way to get out of the phone conversation/meeting. Let your husband know that you don't think he's crazy. Let him know that you understand that he has no control over his parents and you do not blame him for their behavior in any way (something many of us ACoAs struggle with regularly - the fear that others blame us for our parents' behavior).
You can't do much about the parents. They will be what they will be. But you can help your husband. Since he is in counseling and obviously wants to change himself for the better, support that completely and totally. Be careful not to push, he may not be ready to talk about everything with you, but knowing that you're there for him will be immensely helpful (from the 'been there, done that' files). He may initially decline to talk to you, then two hours later open up everything in an emotionally charged manner. Know that any anger or tears or feelings of unworthiness are not things that you can fix, but are things that you can say "While I don't really understand, I'm still here and you are still the most important person in the world to me."
I know your post was asking how you can do things to change your in-laws behaviors. I'm afraid you're stuck with those. But this problem has two sides - I like to think of it like a cell phone: if I call someone but they have no reception, no communication can happen. If you and your husband can get to where you are able to protect yourself so both of you no longer "hear" the messages that his parents are sending, then it ceases to be an issue.
It would probably be valuable (if your husband is okay with this) for both of you to go see the same counselor. You will get a better understanding of his parents and how to protect yourself and your family from it.
You have not described your behavior as anything remotely close to an alcoholic. They are projecting themselves onto you. Your husband has much work to do - be there for him and reassure him that you do not blame him for their actions.
I wish you the best of luck. Welcome to the board and I hope you will continue to post here through whatever problems you're having. There are a lot of us here who know what your husband is going through. While most of us only know it from his angle, we also know what our own spouses have been through while we deal with our own issues.