Well, to be fair, we know how to make both good guns and shit guns. We didn’t forget how to make crap guns; we just stopped making them. If we really wanted to, we could probably make deliberately poor guns intended just for dueling. We could make low-powered inaccurate guns. Make them low powered so that a lucky shot directly to the aorta would kill someone, but even a shot right in the center of mass won’t be lethal. And the low accuracy ensures a low chance of a critical hit.

Now, there are bigger problems with the idea, as you note. But at least the firearms issue would be perfectly solvable through the manufacture of special dueling pistols.