Have only just read this thread. I am a social worker for a young people and family team and deal with situations like this all the time. The one thing that sounds different is that she manages to control her anger everywhere else which makes me think she does have some control. Or perhaps she maintains her anger until she gets home and then explodes?
The fact she is taking her anger out on you more than anyone is actually a positive sign (although I know it won't feel like that at the moment). It shows she trusts you totally and believes you love her unconditionally (although maybe don't like her very much at the moment).
It sounds like she is clinging on to that thought throughout her change of placement. It also sounds like it has been a shock to her? You were right not to give in to the blackmail re needing to come home as the placement is not nice. She will no doubt continue to test your love whilst there.
My advice would be not to doubt the huge influence you are on her life. You know her better than anyone. I would also advise to develop your own plan re her returning home, but not to share that with anyone except family. I would say, don't wait too long, the longer they are in care, often the worse behaviours can get (often due to feelings of rejection and abandonement but also the friends she may meet) so just make sure you maintain contact and offer reassurance and love. Don't go with the temptation of saying if she behaves she can have contact - she needs contact however she behaves. You can always cut contact short.
Try and have different forms of contact - a nighttime phone call saying goodnight every night, taking home for dinner twice a week, lead up to an overnight, then a weekend definately make sure you and her have some one to one time - a walk in the park maybe, make sure contact with other relatives is incorporated - you don't have to be there.. Tell her the rule is you don't talk about what has happened (why she's in care etc) you talk about her and you talk about you - get to know each other again. As you build your relationship she's more likley to open up, talk about what it is that's obviously botheirng her.
Yes, still go down the CAMHS route but don't expect anything to happen fast. Try and ensure she maintains the clubs etc she did at home, they are all very positive aspects of her life. She's likely to feel she is losing everything and is probably scared. She now needs help moving forward, which doesn't have to mean looking at the past. Did you say she has a YOT worker? If so ring them and ask them to do some anger management work with her. What area are you?
Above all else, remember, you and her immediate family are the most effective and important people to help her through this. Make sure you use this period of respite to refresh you all, spend guilt free time together, concentrate on other children if you have them, concentrate on your marriage (if you're married). You all need to recover from some awful draining times.
Try not to concentrate on 'the problem' - try to look towards the future. She may not even know why she has behaved like this, she may feel out of control. Remember,
'Children need your love the most when they deserve it the least'
Good luck, I hope it all works out for you. Feel free to private message me. Zoe X