I like how you're layering on the moments of Andromeda's motivation to run, here. Like it isn't one thing, but many.

also, supremely entertained by the idea of Narcissa as a professional Quidditch player.

I was somewhat confused with the age thing, because . . . OWLS are 5th year, and how is she 17 in 5th year, but then it occurred to me that she would have been held back a year, with a November birthday. But that still leaves her sort of a year off. But that's just a tiny quibble, really!