I am willing to take a nugget of text and squeeze as much information out of it as possible but there has to be at least a nugget of text to squeeze. I just do not see any text to support mortification for any reason other than discovering his judgment was fallacious. I agree Darcy is not a social ignoramus but that does not help your argument IMO. I do believe he understood very well what was expected of him socially at the assembly ball but understanding did not persuade him to behave accordingly. Now that he realizes he judged Lizzy too quickly and wrongly, apparently even before he knew what he was looking at his pride is mortified at making a mistake. Still the knowledge that his judgment was wrong does not mean he will feel or act appropriately contrite for his behavior as a person who has wronged another should. So far, not only has Darcy not attempted anything smelling of an apology he does not even think of anything like regret or remorse which would naturally suggest he make one. IMO his mortification in Chapter 6 is not about his mistreatment of Lizzy or others but about disappointing himself. Darcy is an unrepentant social snob with a predisposition to distain those he feels are beneath him and he has not yet learnt to disparage it. ;D