I've been reading this thread with great interest, but hadn't had time to post. Now you've both led me to a new idea, so I must jump in and have my share in the conversation. ;-)

] Some event brought it all to a head. James knows but only says: "I shall not enter into particulars — they would only pain you more." But whatever it was, it was enough that James was sure Frederick and Isabella would marry.

Right. We must tread carefully here, I know, but I've always thought it was some public show of affection, or some hint at the future within the presence of James or that he discovered somehow. It could even be something as simple as letter-writing, although would James really think that would pain Catherine more? What I mean is, it doesn't have to be the worst thing we could imagine. I rather think it's just enough to make James think that a Tilney/Thorpe wedding was a certain thing, but not enough to make Frederick think so. ;-)

] And I keep going back to that "We parted at last by mutual consent." Isabella would not have done this unless she believed she had Frederick sewn up.

Okay, so here's where my new idea comes in. What if James only thinks it is mutual consent? From the earliest moments of the aquaintance between the Morland siblings and the Thorpes, JA establishes a pattern in which the the Thorpes say things they really don't mean and the Morlands, taking them at face value, misunderstand them completely.

Could Isabella have said something to the effect of, "Well then James, if you do not believe in my innocence then I would much rather not marry you at all", thinking that James could be so foolishly goaded into forgiving her? Or is Isabella too shrewd to take such a chance as that?

] I think there was a lot more going on than we know.

Definitely! As JA says herself when Mrs. Allen and Mrs. Thorpe first become reaquainted in the Pump Room, she wastes neither time nor ink nor effort on tiresome details. ;-)

] And of course, JA wouldn't tell us, but let us imagine or work it out for ourselves. Most vexing. ;-)

Indeed! ;-) Forcing us to use our own imagination. It is impolitic cruelty! ;-) But don't you just love her for it? ;-)