I am glad I can visit the group read today, yours is the very first post I read, and it is well worth reading.

I agree that there is a degree of maliciousness in Elizabeth's treatment of Ann. (Ann, when comparing her to Mary says that Mary is not as repulsive and un-sisterly as Elizabeth).
As for jealousy – I always thought that Mary's jealousy is rooted (among others) in her feeling slighted by her mother. She was very young when Lady Eliot died and had missed having a mother through puberty & pre-puberty.
Reading your post now, makes me think about the effect that it had on Elizabeth.
We don't know, but we can assume that Lady Eliot preferred Ann, or at least spent more time with Ann, since they are similar in character and appearance, as Elizabeth is similar to her father, and had probably spent time with him. We know that at 16 Elizabeth 'succeeded all that was possible of her mother's rights and consequence', but it is probably safe to assume that when Ann returned home she succeeded her mother’s actions and conducts in the village, and had gained similar respect and love that the mother had in society.
I agree with all you wrote about Elizabeth's sources of jealousy, and her jealousy in LR's preference of Ann can also be rooted in the mother's preference.
More about Ann similarities to her mother – some people resent someone who is similar to a loved one that died; especially if they have un-resolved issues with the deceased.