Ok, you're confusing me now, and I obviously don't know enough about your situation to be of any help. You didn't say that your dd had SEN in your OP, and occasionally schools do use scribes when there is no recognised SEN so I satnd by what I said.
She can't write because she can't read You said earlier she has no problem with handwriting and that she would happily and confidently sit down to do the assesment.
And you said her writing is unreadable because she can't spell. Anything. Not due to poor handwriting.
So you can see why I got confused!
I maintain that a child who cannot be understood through what they put on paper, for whatever reason, still has something to offer when it comes to creative writing.
So if she can't read, how do you expect her to spell?
Perhaps that's what you need to concentrate on raising with the school.
I said what I did, partly from professional experience, and partly because my own ds struggles with handwriting. He is 7, and spells phonetically even though he is excellent at reading, and his handwriting is a complete mess. Sometimes to help him with homework, I act as scribe for him while he gets his ideas down, then make him go through what he has 'written' and make him handwrite carefully while prompting him to remember spellings that he does know, but can't remember while he is handwriting and thinking up what he wants to say at the same time.
Anyway, I hope you manage to speak to the school and get the response and support you need from them.