I can’t agree with your assessment of Fanny but I would like to understand your position. (:D) Can you say when Fanny acts like a scared ninny or is spineless? I don’t see a credible suggestion in the text Fanny acts like a ninny as in ‘fool or simpleton’. Edmund “knew her to be clever, to have a quick apprehension as well as good sense” (2). Fanny is timid but not spineless as in ‘without moral force, resolution, or courage’. Fanny arrived at MP:

“Afraid of everybody, ashamed of herself, and longing for the home she had left, she knew not how to look up, and could scarcely speak to be heard, or without crying.” (2)

However within days this lost frightened ten year-old has the inner strength to conceal her sorrows from her relatives:

The grandeur of the house astonished, but could not console her. The rooms were too large for her to move in with ease: whatever she touched she expected to injure, and she crept about in constant terror of something or other; often retreating towards her own chamber to cry; and the little girl who was spoken of in the drawing–room when she left it at night as seeming so desirably sensible of her peculiar good fortune, ended every day’s sorrows by sobbing herself to sleep. A week had passed in this way, and no suspicion of it conveyed by her quiet passive manner, when she was found one morning by her cousin Edmund, the youngest of the sons, sitting crying on the attic stairs. (2)

It seems just surviving the abandonment of her family, except William of course, and life at MP to become an intelligent, kind, loving young woman with a grateful heart illustrates great strength of mind, morality, resolution and courage. She might instead be a broken and bitter young woman inwardly (or outwardly) railing at her situation in life. What acts of rebellion do you expect of an eighteen year old—of Fanny in Regency England specifically? Have you considered how acts of rebellion would be received by Mrs. Norris and Sir Thomas? I am not sure Fanny’s disposition or situation would be improved in any way but I suppose it depends a great deal on exactly what you mean by rebellion. (:D)