Oh yay, I'm so excited for this story! Katharine T's Everingham was eye-opening for me in terms of the potential of Henry Crawford as a character, and I'm really looking forward to your take on him, Fanny, and the others. I actually just reread Bingley's Blunder this morning--giggled and snickered my way through actually--but I'm glad for the more serious tone here. Fanny is so serious herself, and so very vulnerable, that it would be painful to laugh at her.
Mr. Crawford's continued attentions are so frustrating because it either doesn't occur to him, or he doesn't care, that he will hurt Fanny terribly if he succeeds in winning her. In this scenario, he knows his own weakness, that despite "loving" Fanny he has been easily entangled in another flirtation. He seems to assume that being with her will enable him to be true to her--“You could make me into anything.” He doesn't seem to understand that virtue comes by reckoning with your own character, not depending on someone else to continually provide enough motivation toward goodness. But maybe I'm being too harsh--I'm very curious about the change in his tone that Fanny noted.
At least with these letters, Sir Thomas will surely stop pressing Fanny to accept Mr. Crawford, and hopefully feel ashamed of having gone so far as to exile her to Portsmouth in an attempt to force her.