Do you think Darcy offended people because he didn't know what to do or how to do it, or because he thought the people in your examples were beneath him and he couldn't be bothered to make the effort involved in not offending them -- would not give himself the effort, as his cousin said?

The narrator attributes Darcy's behavior at the Maryton Assembly to his judgment that the people there as unattractive, unfashionable and uninteresting and to his characteristics of being haughty, fastideous and difficult to please, not to any lack of expertise in conversation or to his lack of knowledge of what was expected of him in the situation.

Darcy didn't ignore Sir William, he put him down. The putdown about primitive societies was pretty expert and certainly didn't sound like it happened because Darcy didn't know what was expected of him in that conversation.

As for believing he understood Jane's feelings, I think that had more to do, in spite of what he says in the letter, with Darcy's determination to believe that than with any lack of expertise in reading the feelings of another person.

Of course, we don't have Darcy's explanation yet, but I don't think that the change in his behavior when they meet him at Pemberley, where he is socially expert enough to impress the Gardiners, happened by magic. I think he is using social skills he always had but hadn't used very often in the scenes we see in the book, because he was mostly with people he looked down on, or he was struggling with unwelcome feelings for Lizzy.

So I guess given the struggle he is going through and his feeling that he would be degrading himself if he gave in to his unwelcome feelings and married Lizzy, Darcy's poor courtship behavior doesn't surprise me, but what would surprise me would be if he acted that way because he didn't have the skills to court a lady properly or didn't know what was expected of a gentleman who was courting a lady. He certainly demonstrates those skills and his understanding of what is expected once he gets over his conflicted feelings about Lizzy.