Mr. Knightley's anger at Harriet refusing Robert Martin is stated in contemptuous terms, it seems to me. That disparagement might be an immediate response to his own mortification at encouraging a scheme that ends in a man's heartbreak, but it is not at first a calm assessment
I agree that Mr K's outburst of "a degradation to illegitimacy and ignorance" is a bit unkind...Even if soem of it is fuelled by anger at Emma.
Harreit cna't help not being clever or being illegitimate.. and I think that you are right in that both Mr K and Emma while largely holding to the class system as it stood, favoured their "own" protegees and each thought that "their" friend was worthy of rising in the world. That IMO is a natural way of thinking.
I dont think that until he danced iwht H at the ball Mr K thought very highly of her. He would be polite to her but saw her as a silly little thing who was bad for Emma.
But his natural chivalry made him go to help her and then he got to know her a bit better and then he realised that while he woudl still problaby dislike the idea of her trying ot marry a gentleman, he coudl see that in her character she was far better and more likable than Mrs Elton...