] I, for one, completely missed the fact that Colonel Brandon and Willoughby fought a duel the first time I read S&S. Did anyone else have this experience?

I first read S&S for a college class, and the instructor talked about the incident, so it's sure part of my first impressions of the novel. I don't remember overlooking it, but I have a memory like a steel sieve these days.

] Duelling pistols were also not very accurate to shoot, but Brandon is both a soldier and lives in the country so is probably quite handy with a gun. It makes me wonder whether his main intention was to warn Willoughby and defend Eliza's honour with no intent to kill Willoughby.

I believe this absolutely. I think the Colonel probably had his moments when he thought killing W. was "only fair", but I don't think the Colonel is the type to ever have harbored intentions to shoot him in cold blood.

But I believe in the deloping theory. I believe Willoughby knows what he did was wrong, and that he would be able to stand up and admit it, so to speak. That honorable he is, IMHO. Not honorable enough to have refrained from doing it, but honorable enough to know it was wrong and admit it--especially with a reasonable amount of privacy.