With you on not having to rely on out-of-story stuff there really. But it is there at least.

knowing that she'd need to know it at some point in her future

Could be, but again doesn't fit with 'Only time I could'. I really hope we get an answer to that, given as it was such a big point for Ten.

it'd mean she's been in prison since she was a kid

Unless she doesn't kill him until much older.

Oh, or, thought... maybe she's not imprisoned as such, but has not yet been released? The Doctor never manages to save her and she remains imprisoned? (Theory that occurred to me as I'm writing so basis is fairly limited)

I think what this comment should really say is 'I don't know!'