Why then does Austen tell us that Fanny would have married Henry if he had continued ot reform and after she'd had time ot get over the loss of Edmund?

I think that Maria was naturally jealous to hear that Henry had fallen in love iwth another woman and that woman was her unnoticed little penniless cousin... but I dont see that she ran off with himn primarly to keep him from Fanny.. I think she ran off with him because she was in love with him and wanted to be his wife, and she had compromised herself and was unable to remain wiht Rushworth.

and naturally, when she and H lived togehter and he continually reproached her for the fact that their affair had made him lose Fanny, of course she's going ot be even madder and more jealous adn her only bitter consolation, given his lack of love for her, was feeling that "at least F had not got him"....