This part is very interesting to me too. I wonder in the older sages quote if the narrator hasn't slipped in again, because it would be odd if Fanny were to consider whether older sages would forgive these thoughts.

So it seems to me that the narrator has given us two conflicting ideas here.

Older sages would forgive her for thinking Mary can't change.

Experience hopes more for people in love, and impartiality would think that Mary could adopt her husbands opinions and can change.

No, Fanny isn't impartial, but as we've examined so much of Mary's opinions given throughout, it seems Mary would have to make a complete turnaround to adopt Edmund's ideas of clergy religion too.