I am finally thinking that Darcy was just as motivated to get himself away from Elizabeth as he was to get Bingley away from Jane back in November, if not more so. Robbin started the thought by not believing that Darcy really didn't know about Jane's feelings, and questioning his even saying he surmised her likelihood of being in love over the course of one evening. You cemented it by bringing up the proximity of his mentioning himself in immediate succession to his admitting breaking Bingley and Jane up. They were that closely linked in his head.

Perhaps that's what led to the smile of affected incredulity? If he got that Bingley's marrying Jane would bring him in such close, contact with Elizabeth that he would become unable to resist her, then why didn't she get that her marrying him would do the same with Bingley and her sister? Why sweat that at this point? But of course she then starts yammering about Wickham and this point slips away.