I'm really enjoying these continuing discussions too.
I do agree with you, sunsurfer, about Sir Thomas' lack of consciousness of what was going on in terms of the awful Mrs Norris and Fanny. But then I think he was busy with his life and really quite remote from the day to day goings on. I certainly had the impression that he didn't have much to do with the children at all (but then, parents of that class didn't in those times I suppose) and Fanny was rather afraid of him and would have kept out of his way.
Lady Bertram was oblivious to everything that wasn't to do with her and her comfort and certainly just let things happen because she couldn't be bothered. But she did seem to be kind to Fanny in a vague way, and Fanny loved her.
At the same time, if you took Mrs Norris out of the story completely, you would lose the irritation that, as it were, makes the pearl - Fanny is strengthened by having to cope with her. It would all be rather wishy-washy I think.