I agree Sir Thomas’ unfair assumptions and rant is horrible especially as he attacks sensibilities Fanny is least deserving of criticism and most important to her—obliging, unselfish and a grateful heart. I see his point of view but I think he really lost control and that is inexcusable. However he is not without empathy for Fanny. In comparison to his rant his empathy and the kindnesses resulting from it seems mediocre at best but it is there:

He ceased. Fanny was by this time crying so bitterly that, angry as he was, he would not press that article farther.

But Fanny shewed such reluctance, such misery, at the idea of going down to him, that Sir Thomas, after a little consideration, judged it better to indulge her.

There was comfort, too, in his words, as well as his manner, for he began with, “Mr. Crawford is gone: he has just left me. I need not repeat what has passed. I do not want to add to anything you may now be feeling

Check these tears; they do but exhaust you… I advise you to go out: the air will do you good; go out for an hour on the gravel; you will have the shrubbery to yourself, and will be the better for air and exercise.

“I shall make no mention below of what has passed; I shall not even tell your aunt Bertram. There is no occasion for spreading the disappointment; say nothing about it yourself.” (31)

I do not think Sir Thomas’ intentions towards Fanny are unaccountable or he was preparing her to live at the Price level of society in another part of the country. It depends on what he means by mediocrity of condition. It seems you equate it with her parent’s condition and that would make bringing her to MP unaccountable however there is no evidence he ever intended to return her to them or to a similar situation. I think mediocrity of condition simply means that if she married it would not be particularly high and nothing akin to his daughters expectations. This relates specifically to the topic at hand—Fanny has had an offer from a man of more consequence than anyone expected.