ironic that Kan Pishi told Ri Shi to make his mark on history and that the entirety of Qin would rest on his shoulders if they unified it considering how he was a large contributor to the rapid fall of the Qin Dynasty(Li Si irl)
Considering how Riboku falls, I think Youka really is a traitor to Qin and has been working with Riboku for a long time. That arc was meant to build on characters for Han and Youka's eventual importance. He regularly betrays Qin to other states for money. However, he is also Kakukai's follower. So chances are, I bet Ousen already has a contingency for his final battle with Zhao and would blackmail Youka into betraying Riboku. By falsely informing Kakukai that Riboku and SBS will revolt against the king which would lead to his historical downfall and execution.