It's interesting how there's apparently even a bunch of Keiichi centered scenes. I've been actually considering they've been hiding some big Keiichi moments in Watadamashi, after rewatching it and reading the manga version, but I'm not sure what could be happening in Onidamashi.
I think it's possible Keiichi killed Rika in Watadamashi and Mion was just covering for him. He is the last one shown to return to class in the next episode, when Rika had disappeared, and when approaching the septic tank, glanced to the sides before advancing and then seemed surprised the door was closed. He also seemed overly aggressive when Satoko was questioning him about it.
In the manga version of Watadamashi, he's outright going all Onikakushi-like, seeing multiple people with Hinamizawa syndrome eyes like Oishi, not only Mion, being so paranoid that when Mion calls him to Watanagashi the first thing he thinks about is killing people in the ancient ritual, and also HS eyes in some scenes, including right after Rika gives her "your life is over" speech.
It's possible, but Mion had the actual keys to the septic tank, and her first instinct was to go to looking there for Rika in the Neko arc during the hide and seek game. Then there's the fact she stopped Keiichi from opening the door the moment he was about to find the body, which really shouldn't be a surprise to him if he was the one to kill her. Then directly afterwards we get Mion's huge rant about Rika being the one behind everything mirroring Shion. All of that and I figure Mion moving the body without telling him would probably just make him even more paranoid like what happened in Tatarigoroshi.