Sorry this post is going to be long!!!!
The first thing I suggest is becoming aware of options. The second is to recognize that as good a parent (all be it a stepmum) as you are, this problem is way too big to handle without help. It can tear families and relationships apart and basically take over your life. It's no fun for your dsd either! Her drug use is her way of self-medicating some very deep pain, and she will need a three-step process, which must be done in order:
(1) addressing the drug use.
(2) addressing the underlying emotional issues and learning how to manage her without drugs.
(3) learning how to come back into the "normal" world.
Programs which target steps (2) and (3) when step (1) has not been addressed will waste time and effort.
But I guess you already know that and if she won?t accept the help that has been offered to her I don?t know what you can do. I hope she doesn?t have to reach rock bottom before she realizes that she needs this help.
One of two things are happening here IMO (as regards to her 'stirring', she can either sense that you are struggling (who wouldn't stuggle in your situation?) or she is pushing you away so that she doesn't get hurt when she loses you. Of course both could be happening.
I realise that her Father cares for her but kids are funny creatures if they have clear boundaries they may kick against them, then you get the "I hate you" thing, they don't really but its the boundaries that make the child realise that the parents love and care about them. If there are no boundaries they tend to think "I can do what I like, Mum and Dad don't care".
I don't know if I am explaining this too well, but I have known a lot of teenagers who have had problems, there are usually one of two reasons, they are being abused or the parents have set out no boundaries and the kids have no self worth.
It seems to me that you dsd has suffered emotional abuse from her mother ("the bm is an evil person, she tells sd on a regular basis that she hates her and wishes her dead") that coupled with the no boundaries and her loss have cumulated in her becoming this angry, mixed up teenager that has become impossible to control.
I'm sure there is hope for her you have said she can be a really nice girl... and they do grow up eventually, but in the meantime you may be in for a rough ride.
This site may help.
These books may give some helpful advice.
Another site that may have some useful advice for you.
I hope this helps, I am very concerned for you and your dsd. Keep posting.