The toblerones as a hint could've been about Kaan-san, they also could've been about Ashiya-san eating them the week before. If the hint about Yi-kun's shoes and shovel were about someone being bigger than him, it pretty much only left Ashiya-san. For motive too, when it comes to Starphase-san.

[he had a whole thought process and now... he won't know]

.... you don't think Omi-san was a pushover?

[a beat]

.... he just felt entirely like a civilian to me.