This is the biggest and boldest case of a good ol'-fashioned Mary Sue such as I've not seen in a long time. She's ticking all the boxes (a heavily-emphasized heart of gold, tragic pasts and great destinies, and above all, in-this-case-literally magical competence-in-all-things), so far. Her sudden competence is rather crucially the whole point of the series, I guess, but still.
It's not unpleasantly so, mind; rather, this is mostly an exclamation on how nostalgic I'm feeling for, say, the 90's right now. Nor am I accusing anyone of bad writing as such—just occasionally I can't help but laugh a tiny bit. (Mozart coming into play as a magical spirit guide is definitely making me crack up a bit each time.)