Seven years ago, the war between Great Zhou and Dayue was intense, back when Duan Ruo was still the King of Dayue. Duan Mian was in exile within the borders of Great Zhou, and indeed had the conditions to commit the crime. However, he had been dead for many years by then, so it naturally couldn't have been him.
Duan Mian's son, Xu Yin, had just entered the court as an official. He didn't have the power to do these things at the time, and judging by Xu Yin's trajectory in the years that followed, his actions indeed did not intersect with those of King Yan at all.
So, it couldn't have been someone from the Prince Yi Mansion.
It could only have been someone from Duan Ruo's faction.
Duan Ruo was in Dayue commanding the war, so he certainly couldn't have come to Great Zhou Capital himself.
On the contrary, it was very likely to be his son, Lian Yang, whom he had let roam outside.
