I don't understand why he's expecting such dire retribution, such as execution.
He was literally the representative of a Duke's daughter. Duke. That's almost the highest ranking in the noble-hierarchy. If royalty would go for his head, it'd be a direct disrespect to the Dukedom of the kingdom. That's fucking stupid. Am I missing anything here? It's not like the MC tossed a rock at them out of nowhere, it was an official duel, for the honor of an extremely high ranking noble for fuck's sake.
I think a lot of people have a misunderstanding about nobility in those types manga where we deal with an heavily romanticized vision of nobility, laws don't apply to them and honor is the last of their worries as long as they stay in power, nobody will give a fuck if the Prince and his gang send a band of soldiers to slaughter MC family and friends in broad daylight and at the end of the day the word of the king is law and any opposition is criminal, FL family might be insulted but what are they going to do ? start a civil war over a peasant ? nobody is gonna help them for that, heck some of their allies might even seize to opportunity to sell them out for some favor with the future king (the prince) like any good politician.
Nobility in Europe (like most of those mangas are based on and it's the only one i have knowledge about) was nothing like portrayed in media, they had no "honor" or "values", the closest thing they had was religion and all they add to do is send money to the clergy and they'd get a "God says it's ok to commit this horrific crime against humanity" freepass, let's not even talk about knights (and other warrior aristocracy) whose only difference they had with bandits is that one had a state license to act like thug.
Nothing against you but a friendly reminder that Nobility is usually above the law and at the end of the day they could do whatever they want to MC and all people would say is "shouldn't have opposed a noble" and continue their day working to pay for food.