I think this has been an interesting thread, however ,you seem to imply that Darcy has something to apologize for at this stage in the novel.

I can see absolutely no reason to apologize. He made a remark to his friend which was over heard at the ball. There is no evidence from the text that he knew it was overheard.

As for his ‘mortification’ this is just JA using an intense noun for which people of her age would be familiar with the doubly meaning. St Augustine defined mortification in his writings as:

‘St. Augustine says that by denying ourselves certain legitimate desires we strengthen ourselves to resist illegitimate desires. Mortification is an attempt to gain self-control and to free ourselves from minor or harmful things to devote our attention and energy to the things that are really important in life.’

So the reader in the 1800’s would probably recognize the play on words;