] ] in the face of a duel, I wonder (or doubt) how courageous he'd really be. . . Nor is he malicious enough to take the opportunity to really shoot at the colonel.

Oh, he's malicious enough, all right--just not brave enough. His hands must've been shaking.

] To him, Brandon's opinion is worthless anyway, so it's no skin off his nose to concede and save himself the hassle of possibly getting shot

It's interesting to see how consistently Willoughby disparages Brandon in light of, as you say, Brandon's opinion being worthless to him. You're right in that nothing Brandon could say to or of him could persuade him to change his ways, nor could any opinion Brandon holds of him bring him to a sense of real shame. And yet he feels positively driven to tear down Brandon, insult him (behind his back, of course), and try to make that bad opinion of little weight. (More on that later in the novel, of course.) But it seems a classic case of "He who excuses himself accuses himself." In some ways, Willoughby "duels" with Brandon throughout the novel.

] I'm curious who they brought as seconds if any.