I like you're legal analogy that the intention follows the bullet. However, if we follow what jeremy was saying, Darcy's comment would have been made in self-defense (he was defending himself against Bingley's "attack" to dance). Defenses also follow the bullet, so if Y attacks X, X shoots but hits Z; X can still claim self-defense, even though Z was not attacking X. I don't think the bullet analogy quite works though because Darcy intentionally brought in Elizabeth. It wasn't an accidental insult.

I have a hard time thinking that Darcy did not eventually find out that his comments were overheard. Whether he knew on the night of the dance, I don't know. However, if he never came to know that the comment was overheard, then I can understand not apologizing. He would have to explain why he was apologizing, and tell her the comments he made. That would needlessly harm Elizabeth and probably ruin his chance of getting to know her more (assuming, she hadn't overheard the comment).