]5. Jane again. Ah-ha! He chooses Jane because he wants to dance with her, not out of any social obligation. ...Mrs Bennet is right to see this as a sign of his admiration. It's during this pair of dances that he has his significant conversation with Mr Darcy.

Seems to me that Darcy is only attending the assembly out of politeness to his host & party. Had he tried to stay at home Miss Bingley would have done her outmost to change his mind. Is there a particular reason for him to be out of sorts?
Also might one contrive his reply to Bingley "You have been dancing with the only handsome girl in the room" as slightly ...envious? He thinks highly of himself, fastidious as he is, perhaps he ought to be the one dancing with the handsomest girl?( He did think Jane handsome.He even owned as much to Mr Robinson.) And here is Bingley, whose money comes from trade, cheerful as ever, telling him to be content with a mere younger sister, who has been slighted by other men!! The nerve! I certainly shall not! It would be insupportable! I am in no humour at present...! Leave me alone will you!