the neuro debrief isn't about getting a second opinion. it's about explaining the results and diagnosis in terms you can understand. so you aren't quibbling with the results, you just need to know what they mean.
so, in your shoes, i would be wanting to ask:
did the white matter not grow when it was supposed to? when should this have happened during foetal development?
is the fact she was 5 weeks premature because of this brain issue?
is the fact she was 5 weeks premature the cause of this brain issue? (i know you have had one set of docs answer this. i can only say that in my experience, i have had several different answers to similar questions from specialists)
why did it not grow? is there a possible genetic cause?
can white matter continue to develop post partum?
what does lack of white matter mean in an infant? and for her development? (loss of white matter naturally occurs during the ageing process with noticeable and demonstrable results)
what sort of disabilities are likely with lack of white matter? ie she has physical delays - are these 'delays' likely to reduce or increase in comparison with her peer group over time? is it likely she will 'catch up'?
are learning disabilities likely or almost inevitable with this sort of brain difference? (esp as the profs are considering special schools)
is there a support organisation that specialises in this area other than the 'brain' specialists bibic and cerebra? where can i find out more information or get in touch with other families? (note - 'contact a family' here might be of use, but i haven't checked their current lists. they usually have pretty much everything covered.)
what can we expect in 5 years time? 10 years? as an adult?
so, it's really about making sense of the diagnosis that you have received, rather than questioning the validity of their statements. and often we need to hear things repeated, or explained in a different way, before we understand what we are dealing with.
if you have the letter from gosh, i would contact the sec of the dept you dealt with (did you actually get a formal debrief from them - or did they just send a letter to the paed, who 'interpreted' it?) or speak to your paed's sec and ask what would be possible. explain that you are having problems making sense of comments from the physio and paed in the context of the mri results, and would like to understand more about the specific condition your dd has. and ask about dla.
our paed's sec actually got us a copy of dd2's second mri as well. so somewhere i have a disc with lots of pics of the inside of her head... 
have a look at contact a family though - you might recognise some of the terms on their lists.