I absolutely understand your concerns. They are very real concerns and it's an impossible situation you are in.
The reality though, is that as she is 16 unless she is engaging in behaviour that the local authority feel is dangerous (to the point where she could be placed in a residential placement against her wishes) then it is her choice as to where she lives and effectively what she does, even if these choices are highly questionable.
It's impossible, as I said.
You can't enforce the court order for her to return to you as the court simply won't do it - at 16 she can't be forced to live with one parent if she has chosen to live with the other.
I really do feel for you, and I don't want to say that you should give up, but in terms of a remedy through the family court, if the local authority aren't concerned enough to take action there really is nothing you can do I'm afraid.
I expect you were aware of this though.
Is there anything further I can clarify for you about this today? Again, I'm sorry it's not good news in terms of a legal remedy.