OP, I really can empathise. My youngest son could no longer mask at school by the time he was 13. Things have not worked out well for him.
However, the approach that worked the best was by being calm and gentle and trying to remove as many 'demands' (which could be anything that causes anxiety) as possible. This includes not expecting homework, giving the individual a safe place to relax and self regulate at home, keeping the channels of communication open for the individual.
Communication can be difficult with any young teenager, but it may be even harder with a neurodivergent teen in the midst of a mental health crisis. My son began sending me songs when he was slightly older than your daughter. The songs were pretty dark, but they gave me the opportunity to open a conversation starting with, 'Is this how you feel?'. He also communicated (as we all do) through behaviours and keeping calm through this was very difficult indeed. Like your daughter he experienced periods of selective mutism. In fact nearly everything he experienced had its roots in exceptionally elevated levels of anxiety.
Eating disorders are very difficult to support. My son still spends hours in shops trying to decide what he can eat. He has bulimia nervosa, so it is slightly different. I received counselling from BEAT to help me cope with the stress of supporting my son with his ED.
Although PDA (Pathological Demand Avoidance) is a controversial profile, and some local authorities do not acknowledge its existence, the strategies offered by the PDA Society UK (under the acronym PANDA) have been really useful for my son. You can find them on the PDA Society's website.
I am aware that I have rambled, but do explore the PDA Society website.