I am sorry to throw a definition at you but I think it clarifies my opposition. (:D) I don’t see anything in Fanny that is infantile: ‘characteristic of or befitting an infant; babyish; childish’ (dictionary.com). She is shy, timid and anxious at times but that is hardly surprising (as you wrote) due to the domineering and oppressive environment in which she has lived. I also do not see that Fanny is immature. I see no reason to equate an outgoing socially confident personality with maturity.

I really do not see even a tiny bit of a mean streak in Edmund. When Edmund and Fanny are speaking of her moving to the White House he believes Mrs. Norris wants Fanny to live with her and be her companion:

“It has everything else in its favour. My aunt is acting like a sensible woman in wishing for you. She is choosing a friend and companion exactly where she ought, and I am glad her love of money does not interfere. You will be what you ought to be to her. I hope it does not distress you very much, Fanny?”

“Indeed it does: I cannot like it. I love this house and everything in it: I shall love nothing there. You know how uncomfortable I feel with her.”

“I can say nothing for her manner to you as a child; but it was the same with us all, or nearly so. She never knew how to be pleasant to children. But you are now of an age to be treated better; I think she is behaving better already; and when you are her only companion, you must be important to her.” (3)