I agree whole-heartedly with all your assessments of the Mrs. Smith dilemna. She is a complex character, one that we are not given leave to adore without reservation or condemn without at least taking pause to think about her situation.

She reminds me of Benwick in that way. As you say, she could have easily been a 2-dimensional foil, just as Benwick would have, had he been taken a bit further, be really easy to make into a charicature.

Persuasion is a book that often confounds literary critics because in it, JA breaks with her life long career as an ironist, to concentrate on Anne's persona. Anne's reaction to Benwick and Mrs. Smith are the biggest reason they are made into complex characters. If they were more comic/ironic foils (or as you perfectly described, behaving the same way, everytime they are on stage), they would not tell us anything new about Anne. We already know that she suffers the foolish and selfish and that she exhibits a great deal more patience than the reader would with some characters.

"What's interesting about all this is that JA doesn't condemn Mrs. Smith for her ruthlessness -- indeed, Anne and FW remain her friends to the end."