You just explained why Diana was ripe for manipulation . . . but you didn't explain what changed to stop her being ripe for manipulation . . . Particularly manipulation by other people than the vice prez - there are other characters in the story, a decent number of whom probably wouldn't mind being able to use someone like Diana to get at various people, ranging from Lapis-via-Natori to the prince himself. Or hell, even just as a weapon - she's powerful, and she's only going to get more powerful as she's exposed to more and more spells and gets more and more experienced.
I'm genuinely curious about your views on how Diana has grown beyond being manipulated by anyone, ever, under any circumstances . . . .