IIRC you are wrong about failing a single perception check locking you out like that.
IIRC if you fail the check you can still manually investigate the site of the murder and get the proof either way.
I didn't need to be an omniscient player to know Dasan is a POS. He was blatantly obvious about it.
And no matter how many times you say it is "most logical", it won't magically make it so.
Letting a coward who slew his own father take over so he can sell his own people as slaves becoming chief is literally the polar opposite of logical.
All I can say is that my MC didn't find any proof there and I did even search the corpse, in later playthroughs with percepcion I was able to find the proof.
I do agree that the most logical option if you find the proof is to banish him, however if the MC didn't find it then killing Dasan in a duel feels too much, albeit to be fair I just feel that stuff to be a bit too rushed. Personally I would have liked a development more in these lines:
1.Dasan goes together with his father, but rather than fighting he ran away and his father died.
2. Dasan becomes the chief and tries to surrender to the orcs.
3. The village is divided and some warriors leave to fight the orcs, giving the player the option to join the warriors or stay in the village.
3.A The player joined the village warriors and in a great effort managed to defeat the orcs and come back victorious, surprising the village and Dasan who didn't think it was possible.
3.B The player stayed in the village, the warriors died and the village surrendered to the demands of the orcs. Dasan feels that he was right.
4. The events change depending on the route, in the first Dasan falls into self-doubt about his choices and his doubts will keep growing with each victory you achieve, to the point of ending with Dasan deciding to admit that he was a coward in front of the village and giving up the chieftain position, this would be a route in which Dasan redeems himself and becomes a positive character, trying to overcome his failure and cowardice and become a true warrior of the village.
On the other hand if the warriors are defeated Dasan will feel that he was right all along and will become more depraved, abusing the power of the chieftain and in this route he will try to cuck the MC, Nyra would try to rectify him and to wake him up but it will ultimately fail, basically arriving at a point in which she asks the MC to defeat Dasan before it's too late and you having the option to either accept it or ignore it, just letting Dasan do what he wants would be the full cuck route.
Overall this is my personal overview of how I would have liked things to happen.