" Considering that she saved our asses, can we even set that boundary? "
This is not a traditional landlord-tenant question.
You cannot just start saying stuff like "Under LL/tenant law, you are required to give me 24 hours notice..."
They are likely to reply "In that case, we will expect full market rent and will evict you if you are more than 3 days late, in accordance with LL/tenant law".
Nope, don't go there.
I have to take the parents' side on this one. I actually am good friends with some parents who are doing exactly the same thing your in-laws are doing. They own a small house, and are letting their adult daughter and her two children live there for almost no rent.
My friends do the same things as your in-laws. They go over there whenever they feel like it. Why? Because they don't want their daughter to get too comfortable with the situation. They really want her to earn enough money to get out and get her own place, so they can either sell the house or rent it at market rent prices to someone else.
The daughter complains all the time about her parents dropping in at any time of the day or night, whether or not she is at home or not. They have their own key and just let themselves in. The parents' attitude is "Tough. Don't like it? Move."
I am not saying that your in-laws have the same point of view. However, your life is now rough because of the DUI. Your husband made the decision to drink and drive. He has to face the consequences, and unfortunately, so do you. One of those consequences is a huge amount of inconvenience and associated costs.
Had he made the decision to not drink and drive, you might be in your own place by now.
Sorry to be so harsh, and I know it's tough for you, but my best advice is to just suck it up and do your darndest to get your own place.