Another really fun arc indeed so far. Im also betting they are all the suspects, Usogui played to win. Honestly, sometimes its confusing what kind of things Kakerou allows or not. Yakou seemed to imply the mine game with Ikki Sadakuni (which was set up from the start with the eye cameras) was not allowed by Kakerou standars, but this one is also totally set up and even though Midara admits its fishy he allows it. Though I guess Yakou was more concerned about the mine game being used numerous times.
PD: Thank you for the data on the no-pan kissa and on chapter 68, gonna check them out.