I don't think looking out for the safety of your children should ever be considered codependency. Their very lives could depend upon you doing just that.
I agree that being assured that she would not get custody in a divorce is crucial. There's no point in leaving her if she's going to have the children alone more than she does now. You do have to put them first, even before yourself perhaps in this case.
I also think a strict boundary must be in place before she comes home again. One drinking episode with her alone with the children, and all bets are off. She too must recognize that their safety is paramount.
It's very tough love, but this is serious business, especially if she gets behind the wheel with those kids in the car. Unacceptable on every level...and potentially deadly.
This is one of those cases where ultimatums apply. Don't you ever feel guilty for trying to keep your kids alive.