I have to admit that I don't think the explanation of the theory I mentioned explains all details of the family. Your points are well-taken. There are so many facets to every person, and I really think that even the caricatures that Jane Austen wrote, have complexity to them, such as Mary. Would Mary as a character have worked if she was placed as the youngest? Would she have worked if she were the oldest and say, Lizzy were the youngest? Would the family dynamic work if Mr. Bennet were a more engaged father? And Mrs. Bennet less silly? Or if there had been a brother? No adaptation has ever removed any of the Bennet family that I can recall (unlike in S&S, sometimes Margaret is entirely forgotten.) She has her place, and I somehow I do feel like it is appropriate that she's the middle of 5, and that she hasn't received attention from her father or her mother, and she is the plain one, so she tries to make for that without any guidance, and it fails miserably. And she probably feels like she's competing with her sisters, having no advantage over any one of them, and so she's also nor good friends with any of her sisters either. Tough spot! If she did have some witty nature, such as Lizzy has, or an affectionate nature like Jane, she might have befriended her sisters, and the negligent parents wouldn't have mattered so much.