I don't think it was a big mistake at all. As those before me have said, Alexandra is very active in this chapter, fighting through incredible circumstances to have reached the point she has gotten to, it's only the first part of a series, and it's certainly far more realistic and logical than the way Harry and his friends beat Quirrelmort in the first book (author fiat and deus ex machina).

Rowling's system is so flawed, I think anybody relying on 'canon' is going to wind up with far too many plot holes and inconsistencies to make a reasonable story.

But what most caught me was this line: "We watched him place the circle around your little wrist, before he sent you off with your mother.” But in Abraham's telling of it (and in Diana's as well, I might add), Abraham and Hecate were caught together: "Then everyone went their separate ways... Except Hecate and I, of course, and our daughter." So... a deliberate piece of misinformation you planted all the way back there, or just something that needs to be edited?