Yes, one could almost miss that the Colonel and Willoughby fought a duel, unless one is familiar with some of the euphemism surrounding the practise. I think Brandon's circumspection has more to do with his character, than fear of criminal charges. He doesn't reveal a lot, and would be especially reluctant to blurt out that he took a shot at Marianne's suitor.

As so often happens, JA gives us a few clues about events "offstage", prompting a great deal of speculation. It is very unlikely it would have been swords: first, because pistols were most comman in Britain; secondly, Willoughby (as "challenged" would have choice of weapon and knew he would have very little chance against Brandon using a blade.

As mentioned, pistols were very inaccurate, especially with the "smoothbore" type with an "unpatched" ball (the musket ball would rattle down the barrel, going off who knew exactly where) over a distance of fifty feet (assuming the twice-ten paces). While Brandon would be more familiar with a pistol, it does not necessarily follow that he would be a good shot with one. It really isn't a battlefield weapon (except perhaps in assaulting a breach), and many declined carrying it as being of little more than excess weight. (Plus, an officer's job is to direct the company or battalion, not adding to the general din.) Thus, it's likely Brandon might not have practised much with one.

How the duel went itself also leads to a greater deal of speculation. Sometimes the shots were exchanged simultaneously upon a signal, other times the challenged got to go first. (I suppose B. likely granted W. the latter protocol.) This leads to what Willloughby did. Did he aim, fire and miss? Or might he have shot wide deliberately. This could have been done out of some moral feelings (guilt, not wanting to kill another human, etc.)

I think there might be another possibility - a bit of a gamble on Willoughby's side. He knew he'd probably miss if he tried to hit Brandon, but the Colonel likely was a better shot. But, if the purposely delopped, he then forced Brandon into either returning the favour, or somewhat callously shooting him. On the other hand, both might have done their best to do in the other, but technology failed them.

I remain,
Jason E.