Thank you so much for your reply - it's so kind of you to take the time, I really do appreciate it. I've looked again (and again!) at the links and hope I've written a much more focused letter, taking into consideration your advice.
There is no corroborating evidence at all for either of my extenuating circumstances, but my feeling is they certainly affected her performance on the day. Maybe the school will feel she/we are too ambivalent, in light of the rows? My parents weren't wrong when they used to say "Just you wait til you have children...!"