Yes, I think we are attracted to the extraverts like Elizabeth Bennet, but Fanny is clearly an introvert (I found a sentence about her enjoying her own thoughts and company, but left my notebook down at our beach house so don't have it to hand). I suppose what Jane Austen has done is put "the poor relation" in the centre of the story, instead of her being pushed to one side and barely looked at, which is what usually happens to poor relations. And of course Jane Austen, her mother and sister would have been in that position if Jane's brother hadn't been adopted by their rich relative and was able to give them the house in Chawton to live in.
I hadn't come across any comments about slavery and Mansfield Park, and had not had any recollection of its relevance from reading the book way back when. I suppose I was pretty ignorant of the times and what was in the background when I was in my teens, so just hadn't picked up the references.