T.C.
I am going to be very frank with you with all of the love from my heart.
Watching and allowing your daughter to drink underage is unacceptable and irresponsible. Knowing that she is drinking while out and coming home drunk is allowing your daughter this freedom. It is time for you to have your daughter arrested. I'm sure you find this harsh but your daughter needs to understand that you are serious about her underage drinking.
I can tell that you love your daughter very much and are concerned about her health, well being and safety. I promise you, at this point, your empty threats will mean NOTHING to her. You allow her to live in your home with no consequences to her actions.
1. You cannot keep her from drinking so much when she is out. She is eighteen and believes she knows everything.
2. Why do you care so much about being authoritarian? You are the parent and she is the child; no matter how old she is.
3. She IS still a child. If she is living in your household, she needs to obey your rules.
I'm sure she is a great kid -- she knows you'll accept her - drunk or not. Lay down the law and get a backbone.
I am completely coming off very harsh; I know. I only want the best for you and your family and going through this myself - I can tell you that wishy-washy parenting will not suffice. Hopefully your daughter has a lot of great things to fall back after she gets through this, but for now - tough love, counseling and new friends are the most helful solutions.