However, as a general comment, I think it is difficult to draw general conclusions about Darcy's communication/conversational abilities based on what is probably the most atypical period of his life.

So we can't infer anything from the only evidence we have? Maybe. But what, besides running into Lizzy where he least expected someone like her, might be so unusual? We know the Bingleys are even more inferior (in social status) than the Bennets. We know Caroline is a constant irritant. We know Lady Catherine is an even worse snob than he is.

Also, just a couple of examples from your latest post on Darcy as a conversationalist ... not a likely description for a man who "who can't..."

Well, maybe. On the other hand, at Pemberley he was completely tongue-tied just a while before; he fled for some considerable time, giving himself time to prepare his addresses. And he's on his own turf, directing the conversation toward safe topics, and talking with people who are themselves easy conversationalists, guests who are certainly not going to challenge a man in his own front yard, who will accept his lead in the conversation.

And again, at Gracechurch Street, he's had lots of time to prepare his speeches, he's directing the topic, he's leading the attack on a subject very dear to his heart.

Those shows of skill (imo) came from intense preparation, high intelligence, leadership of the conversation, and intense interest in directing the outcomes.