a littleone2
I would also put your post onto the Special Needs forum of this website as you will get replies that way too.
School Action Plus in the circumstances that you describe for your DSD is not worth the paper its written on. It is not legally binding and is very limited in its scope (it will not address toileting problems for instance). Her needs as they are go way beyond what SA plus can do - she needs a Statement (legally binding document obtained from the LEA).
Her birth parents are doing her a huge disservice here by burying their respective heads in the sand. Denial is here and it is a powerful force. Your partner needs to be forcefully told it will not all go away by denying her learning difficulties. Labelling is the signpost to getting the child more help - he needs to look at it this way. It will get worse for this child the further she proceeds through the school system without support (she could end up getting permanently excluded). You are right in thinking that a tutor will not sort out her issues.
From your description of her difficulties as well she needs to be seen by a developmental paediatrician as a matter of urgency with a view to getting a diagnosis. The GP can refer you to such a person. Her birth parents (at the very least your partner) need to become more nvolved if this is not going to end badly in terms of her schooling.
She also needs a Statement of Special Needs with regards to her education because currently she is being sold well short here by the school. It is clear to me that they cannot deal with her whatsoever. A Statement is a legally binding document which provides the school with one to one funding for her.
It would not surprise me in the least to read either that no-one has mentioned the "Statement" word before now.