...on the businesslike Miss Thorpe's part - the engagement to Captain Tilney never happened! ;-) Actually, now I come to think of it, I wonder if her relationship with Frederick wasn't a little like Mrs. Clay's and Mr. Elliot's in "Persuasion". Frederick was obviously much more a man of the world than James. Besides the fact that he had better prospects than James, I wonder if his flattery didn't go to Isabella's head enough to make her do something against her own self-interest, ditching one fiance before she was absolutely sure of the next! (And yes, I think that even if she and James had quarrelled, in this case convention was on her side. If she had sent for him the next day, apologized and announced that she considered herself still engaged to James, as an honourable man I think he would have accepted it, even if with a heavy heart, so I think the onus was on Isabella to maintain or break the engagement she was already in.)

P.S. to Adrian: as you point out, the fact that Isabella was already dancing with Captain Tilney before she heard the news about James' finances, seems to me to show that she already found him attractive.