I agree that Darcy needs to become more civil to those outside his circle, as part of the larger process of becoming less prideful that every Darcy has to undergo. I enjoy reading stories, like this one, where he and Elizabeth are able to recognize their faults and work through them (even to an extent together) without enduring the harshness of the usual Hunsford scene.
The scene at the play reminded me of this: "He looked serious, as usual; and, she thought, more as he had been used to look in Hertfordshire, than as she had seen him at Pemberley. But, perhaps he could not in her mother's presence be what he was before her uncle and aunt. It was a painful, but not an improbable, conjecture." I had been thinking that perhaps Darcy could not in society be what he was in a more private setting, and that Elizabeth was reprimanding him for that. But in fairness, I can't see the facial expression that you imagine or that Elizabeth sees, so perhaps he actually is glaring at these people rather than just being reserved. Even in that case, I would take issue with the way she reprimands him, but she may be less at fault than I was assuming.
