Shirley, a couple of observations...I hope I am not going to make matters worse as I know there are no real assurances from any of us until you are through the other end!
Your DD is in a prep, is bright but has slow processing (at this point in time). Still in Oct, and this will get better as she gets more familiar with the format of tests/questions/where to seek out the marks etc. I hope your prep is focusing on these.
As your DD IS bright, your prep head's report will be important. I know some schools will make a hard line cut off, but for others there will be some arbitrary potion mixing that will include the wider profile of the pupil, including the reference that you are fortunate to have vs others from non-preps/state that may not be so comprehensive.
I am about to contradict what I have said in other threads that only you as Parent can judge how many exams to sit is right for your child, but I am slightly concerned at the creep increase you have (and you haven't reached Nov application cut off yet!). 7, if individual sittings, are a lot for a child to sit, especially given the added pressure on her processing speed. Fatigue may kick in on the latter exams which will be counter-productive and could leave her with a negative view on the whole thing.
Is it your Prep head who is asking you to cast a wider net or your view to? I would keep engaged with him/her and agree where the gaps are at this point in time and how you can both cover so she grows in confidence from now to end Dec, not destroy it by not feeling good enough. Let her innate intelligence come through on what she can answer well, not on what she misses. Focus on her comprehension as an example where long answers are not required. How creative can her writing get, even if she cannot complete the recommended no. of recommended pages...etc.
A good prep school rarely fails to get a child into a well matched school as cake has really usefully demonstrated, so get some honest feedback from them now on a joint strategy to work towards.