I agree that this is *not* a pleasant smile! I find this one a bit hard to decipher too, but I *think* the "affected incredulity" is because Darcy can't believe that his breaking up Jane and Bingley would be enough to make Elizabeth refuse *him*, and he's waiting to hear what her real reason is (e.g: she's just upset about the way he asked her, and is still planning to say yes eventually - shades of Mr. Collins?)