Problem is that once a new King is crowned, their power is at their lowest, specially when no one swore fealty to them yet. Add that the MC is technically a peasant. Acting as an autocrat from the very start SHOULD have serious consecuences unless plot armor strikes, and what is he gonna do? Execute the majority of nobles that will see him as a tyranic usurper that somehow wormed his way to the crown? Thats the perfect recipe of having a civil war on which the country will destroy itself. Winning a war is not the same as ruling a country, and even if some mentions Machiavelli ideas do remember he did fell in disgrace and died forgotten by his peers
Before a kingdom exist, every 'king' was a 'peasant'. Guess what do they usually call someone taking down a tyrant like Dibel? a hero.
Previously you suggested to take them out slowly one at a time, but did you not consider that maybe once a few of them die the rest will realize what's up and start a civil war anyway? With more time to prepare they'd cause a lot more damage even if he win.
The civil war was inevitable as long as Dibel is alive, all your 'this scene is silly' reasonings pretty much ignored what happened in the story so far (including the many times we've seen Dibel faction being chipped down from the 'considerable influence' they had at the start)
Like, did YOU even think about the scenario you suggest? We've seen multiple times what kind of person Dibel and his faction is, do you somehow seriously think they wouldn't eventually cause a civil war?
They can just defend themselves by arguing that they were accused wrongily so that the new tyrant could get rid of them. Trials take time and you dont need too many nobles to believe them and rebelling against the usurper to get a civil war in your hands
Except the testimony was from prince Carlo. You know, the one who was invading the country JUST YESTERDAY?
If they refute that, they're accusing a prince of Domorgar of lying.
Another thing: While going back to check there's the chapter where Bernhard's health plummet after he lost Blessing of Kings benefit from Rigar (also Bernhard sent a letter to Domorgar king, so there's solid evidence)
So Blessing of Kings not only increase the king's power but also his subordinates. Once Almus remove Rigar he'd be the only one left in the country with that blessing, so there's even less reason for nobles to even think about antagonizing him.
EDIT:
https://mangadex.org/chapter/88ee6236-b178-4b36-a3bc-75f0d54280be/11
Apparently Almus's BoK is an evolved one, so once the news that his version buff his subordinate come out, nobles would be scrambling to bow to him if anything.