Zhuge Xiong's conjecture was very possible. There were truly some powerful cloning and avatar creation techniques here in the Supreme Ancient Immortal Realms. Even if the two are separated by distance, they could still appear simultaneously for a long period of time. There were even more heaven-defying ones where the avatar could possess the same combat prowess as the true-self.
They didn't think of this earlier because this was simply a dead angle. When the Jialan Monarch died, it was very natural for Qin Wentian to appear in his governor's manor. No one would suspect him further.
However, Zhuge Xiong was exceedingly intelligent and was always the one thinking up schemes and devising strategies for the Lifire Palace. He was highly regarded by the Lifire Empyrean. After rounds of investigation, he discovered that there was no way to continue investigating the Jialan Monarch's death. This was an unresolved case, destined to be unsolvable.