] Also the fact that both Willoughby and Brandon were 'unwounded' in the duel suggests that it must have been fought with pistols. ... 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.

Yes, I imagined pistols. And I cannot imagine that the Colonel, being a soldier, would miss what he aimed for. So my speculation is that Willoughby "deloped", ie shot into the air which was an admission that he was at fault, that he accepted that he did wrong. Brandon, being the honorable man he is, would have afterwards done the same, for he could not then shoot an unarmed man, just standing there.

I also wonder, if Willoughby did acknowledge his culpability, if he then did anything toward Eliza and his child's support?

] but on the other hand having killed Willoughby would likely have turned Marianne's indifference toward him into hatred!