Your post is a great summary of the youngest Dashwood sister from what information we get of her so far, more detailed than anything I can recall seeing on her. As I was skimming through my Cambridge Companion to Jane Austen for something my eye caught the following:

"...(S&S) resembles the didactic novels by Mrs. West and Maria Edgewood that were published around the time it was drafted, which compare and contrast 'the beliefs and conduct of two protagnoists---with the object of finding one invariably right and the other invariably wrong'. The presence of the third, youngest Dashwood sister, Margaret, hints at Austen's intention to diverge from this pattern. Sense and Sensibility corrects the typical didactic emphasis by refusing to choose between Marianne and Elinor."