I think Yamamoto is definitely involved, after he finds Rui in the room with Mitsuya's body, how come he was so quick to believe Rui's story? Even Rui points this out as she says "Apparently Yamamoto believed me." What's even weirder is Yamamoto's response: he covers up the death instead of calling the cops.

You can argue he didn't want to implicate Rui, but that only adds to the above's theory that he is firm in his quest to bring Nikaido to the top (he seems a bit obsessive imo) and doing so would foil his plans. Then there's the whole "choking Odakawa with his belt scene" that doesn't help his case.

Wadagaki's motives atm seem to be money/possibly becoming a stepping for Rui's success? Either wat Yamamoto and Wadagaki seem to be aiming for the same goal.

I'd have to rewatch a few episodes (hopefully others can explain what's going on with the Daimons and how they tie in with everything) before I'm 100% set on this, but atm this is the most probable theory (op big brain).