Sorry things have got worse. In getting help, things do sometimes get worse before they get better and I imagine you DD is in a very frightened place as the structure of life/school for her is falling apart and she is desperately fighting to stay in control.
I would advise from my experience, not talking to her about not doing exams until a professional is more involved, just know in your head it wont happen. All you can do, is say, we will get through this together, keep telling her you love her (even if she doesnt respond) tell her lots of young people struggle at this age etc. If camhs do recognise why her behaviour is as it is, when she feels stronger I don't think she would benefit from returning to a school which has been so punitive. All I can say, is that in the scheme of things, going back a year and redoing it in a new college turned out to be an easier part of the journey we faced.
Hopefully camhs recognise her distress from her reaction, even if they left her screaming. If she is hurting herself like this, make sure you phone and email them, letting them know of her behaviour and if you think she is in danger, call them immediately.Keep a very close eye on her.
write down as much as possible of what is happening and know all the names of people to contact 24 hours a day. (even higher up in the NHS trust).
Diagnosis ultimately might not be that easy (we have had lots of different ones over several years) and they take time, the important thing is at the moment she can access support, and engage with it. Take things day by day, read up on interacting with an ASD teen and anger (even if she isnt, parenting skills suggested will be helpful) and do something , however small, each day, just for you.