. . . Many people have had power over Fanny Price: her mother who gave her away, her aunts and uncle who make it clear that she is not a "Miss Bertram," the Crawfords who annoy and alarm her.

I am having trouble identifying people over whom Fanny has power. Yes, in the pivotal "East Room" scene, Fanny refrains from exposing Henry Crawford to her uncle. It would be true power if, like the emperor in your example, there would be no repercussions on Fanny destroying Henry's reputation. No one would dare gainsay a ruler who condemned someone to death. They wouldn't risk ending up on the chopping block alongside the first victim.