Interventions can and in some cases do work. Just like rehabs sometimes it works and sometimes it doesnt.
The big X factor here is the addict themselves. If they are NOT ready to quit they wont. Just because an addict is OFF of the drugs doesnt mean they are in recovery. Recovery is about finding out the why's. Take away the drugs and you still have a damaged person.
So I hear you looking for answers and trying to figure it out. We all do at some point. But really in all cases with an addict they are NOT gonna change if they dont WANT to change.
Interventions can work but all of the people involved have to be strong and healthy enough to say to the addict enough is enough. Most codies are just not capable of this without time in their own recoveries.
I am sorry that you and your daughter are going through all of this. The best thing you can do is let her hit her bottom on her own. Give her the respect of living her own life whether you agree with it or not. I am not trying to sound harsh here but your job is to save your self and get yourself better.