Thanks Stephanie for your kind words—although we do not always agree I enjoy the discussion! (:D) I agree Mr. Knightley was “mortified to have been the means of promoting” (8) Mr. Martin’s offer but I disagree he speaks contemptuously of Harriet for her refusal. His blame for her lives no more than a few moments. His strongest provocation and condemnation is for the “the part which he was persuaded Emma had taken in the affair” (8). His initial reaction to the news is shock, followed by displeasure and indignation:

This was obliged to be repeated before it could be believed; and Mr. Knightley actually looked red with surprize and displeasure, as he stood up, in tall indignation… (8)

At first Mr. Knightley blames Harriet’s silliness for the refusal saying "she is a greater simpleton than I ever believed her. What is the foolish girl about?" (8) which suggests he is disappointed in Harriet. He thought she had more sense than to refuse such an advantageous offer and he was right. Harriet intended to accept until she realized it did not sit well with Emma. When he realizes Emma is the author of all his indignation and displeasure with Harriet gives way to anger at Emma: “Emma, this is your doing. You persuaded her to refuse him” (8). Mr. Knightley is angry at her interference but also her foolishness:

"Not Harriet's equal!" exclaimed Mr. Knightley loudly and warmly; and with calmer asperity, added, a few moments afterwards, "No, he is not her equal indeed, for he is as much her superior in sense as in situation. Emma, your infatuation about that girl blinds you. What are Harriet Smith's claims, either of birth, nature or education, to any connection higher than Robert Martin? She is the natural daughter of nobody knows whom, with probably no settled provision at all, and certainly no respectable relations. She is known only as parlour-boarder at a common school. She is not a sensible girl, nor a girl of any information. She has been taught nothing useful, and is too young and too simple to have acquired any thing herself. At her age she can have no experience, and with her little wit, is not very likely ever to have any that can avail her. She is pretty, and she is good tempered, and that is all. My only scruple in advising the match was on his account, as being beneath his deserts, and a bad connexion for him. I felt, that as to fortune, in all probability he might do much better; and that as to a rational companion or useful helpmate, he could not do worse. But I could not reason so to a man in love, and was willing to trust to there being no harm in her, to her having that sort of disposition, which, in good hands, like his, might be easily led aright and turn out very well. The advantage of the match I felt to be all on her side; and had not the smallest doubt (nor have I now) that there would be a general cry-out upon her extreme good luck. Even your satisfaction I made sure of. It crossed my mind immediately that you would not regret your friend's leaving Highbury, for the sake of her being settled so well. I remember saying to myself, 'Even Emma, with all her partiality for Harriet, will think this a good match.'" (8)

I don’t read contempt for Harriet in what Mr. Knightley says of her (above, unbolded) because it is merely the facts fuelling his appalled consternation at Emma. He is trying to make Emma see reason. He questions Emma’s powers, recites Harriet’s claims to show why Emma is wrong and ends with his mistaken confidence in Emma’s judgment. IMO the paragraph (above) is about Emma’s supreme colossal error rather than a diatribe on Harriet’s qualities. Of course Emma finds the truth slighting of Harriet but she is “blind, partial, prejudiced, [and] absurd” (P&P, 36) in her estimation of Harriet’s claims. I see Mr. Knightley’s counters to Emma’s further arguments for Harriet’s claims in the same context:

"Upon my word, Emma, to hear you abusing the reason you have, is almost enough to make me think so too. Better be without sense, than misapply it as you do."