Did anyone ever have to watch the show in school "Cipher in the Snow"? A little boy dies and no one knew him.

Fanny is a cipher at Mansfield Park. She was taken from her family, where she was important to her brothers and sisters, and at Mansfield Park she is important to no one, except Edmund once he begins to take an interest in her.

I agree with the reasons you give Jeffrey. Sir Thomas inspires awe in even his own daughters, just like Fanny, so none of them are comfortable around him. Again Lady Bertram is not mean, but she just doesn't care. Poor Fanny is the Cinderella to "evil stepmother" Mrs. Norris.

Fanny's only love in the world comes from letters from William and Edmund's friendship. So when Edmund isn't there, she has nobody. Do you think if Edmund fully understood how much Fanny's happiness were tied up in him, that is would be a daunting prospect?

I think of Edmund as a pretty stand up guy. Not only does he feel Fanny must have a horse, but he's takes full responsibility to make it happen, by trading his own horse, rather than relying on his father's money.

Edmund does get distracted and neglects Fanny. All people can be thoughtless at times, and it's not surprising that Edmund is distracted and forgets Fanny when preoccupied with Mary. It's funny that Fanny stands there and watches Edmund and Mary riding, her jealousy is practically palpable when she thinks She could not but think, indeed, that Mr. Crawford might as well have saved him the trouble; that it would have been particularly proper and becoming in a brother to have done it himself;.
What she's thinking about Mr. Crawford is probably true, but her train of thought reveals a lot of her feelings in that part of the story.

When they all (except Fanny) go out riding four days in a row, Fanny is left basically as servant to Mrs. Norris stuck responding to her every whim. I guess what I'm saying is because Edmund is the only who seems to truly care, when he does neglect her the importance of that neglect is amplified.