Possibly this should now move to the NA board.

Myself:
] Isabella evidently believes herself engaged to Captain Tilney (otherwise she would not have broken her engagement to James Moreland) yet he (Cpt Tilney) 'escapes.'

Caroline:
] I disagree... I think she breaks her engagement to James in the hope that Capt. Tilney will propose.

I based my presumption on the following statement of Henry Tilney's:

"I have too good an opinion of Miss Thorpe's prudence to suppose that she would part with one gentleman before the other was secured. It is all over with Frederick indeed!"

This does allow some ambiguity - your view would have it that she broke the engagement because she believed she had Cpt Tilney's heart secured (but proved to have been deceived.) My view has it that Henry believes she would not feel a gentleman 'secured' without a proposal.

If Henry did not think that 'secured' equalled 'engaged', then I think his next statement implies he thought Frederick incapable (whether by morals or acting ability) of so deceiving Isabella.