compassion,
I know you are trying your hardest. And that your husband is confused about what to do - and is giving in to her.
It is INCREDIBLY HARD to break an addiction. And it is almost impossible to do it if you are in the same environment you used in, esp if you don't really really really WANT to change.
It certainly doesn't sound like your difficult child WANTS to change. I would be surprised if she wasn't sneaking some substance or other, esp at those concerts and parties your husband takes her to. It is WAAAAY to easy to use and then hide it, esp if you have a parent who gives in to you in spite of behavior contracts.
As often as your husband has given in to her requests/demands that are out of bounds with the contracts, the chances she is mending her ways are slim.
For HER sake, a dual diagnosis facility (treats sub abuse and mental illness at the same time) or at the LEAST a rehab facility is what you are going to need to do.
If you leave info about the places laying around, esp a transport contract, she will most likely run. She WON'T see it as a chance to clean up her act so she doesn't have to go. She will just run.
Make plans, but make them in secret. When you are ready to put her in the car and drive her to the facility, then tell her. Or tell her once she is IN the car and teh child safety locks are engaged. Or the transport firm is there.
I talked to my son before I posted this (Wiz) - using only VERY general terms. He said that he saw kids at the long-term psychiatric hospital he was in that knew well ahead of time they were coming (even the night before is well ahead when I pinned him down on the time frame). These kids, every single one, RAN.
Their families had to search for them and PRAY that the child was found before the bed was given to someone else.
Make phone calls when she isn't home, or when you are somewhere else. Keep all the notes you write hidden or locked up.
And if you feel she will rage in the car once she finds out where she is going, then have the police or a transport company take her there. You can always follow them.
No matter what, make SURE that there is another ADULT in the vehicle with you - one who can subdue her if she starts to grab the steering wheel, get out of the car, etc.... It will also help YOU to have them there, as this will be an incredibly difficult thing to do.