] Though it is not all Fanny's fault - JA does tell us that John is "cold-hearted and selfish", Fanny is just more so. And though he might have improved with a better wife, he is still naturally disposed toward being a rotter. ;-p

Exactly. Every time I read this chapter I get the feeling that John was quite content to be manipulated this way, having Fanny supply him with suitable pretexts to justify his retraction of the promise to his father.