Thank you both for your input, Tom P2 and Barb JA, you have given me plenty to think about. I'm still not sure where I stand regarding the roots of Fanny's humility, but I'm in no doubt whatever about her inner strength.

You make some good points about Fanny's 'reflections' Tom. I'm not sure how they play into the ambiguity I was so incoherently attempting to pin down, but it looks like a promising angle to pursue. I may be wrong, but I can't think of another Austen character where we are shown their internal thought processes in quite such forensic detail, from instinctive reaction through doubts and reasonings to a decision. I have been focusing on other aspects of the text, so have not really looked at this in detail and now is not the time to start. I might take this as a theme for the next group read of MP.

Since posting I've been chewing the gratitude question over some more. I agree that Fanny is not "improperly grateful" (ie grateful because she ought to be). Her gratitude is spontaneous and unforced, which is why accusations of ingratitude from Sir Thomas, Mrs Norris and Edmund-the-dolt are so woundingly unjust.