Decided it's better to ask here, as the story discussion forum seems dead. Obvious spoilers below.

So you're telling me that Okiura is able to receive a birth certificate for an unknown baby, and yakuza are able to buy military-grade equipment (like the bulletproof Kumakura vehicle), but none of them are able to cremate a body? So OK, maybe Kumakuras decided to bury a body in the forest to cover the crime, that kinda makes sense. But then Okiura decides it's better to place a body in a cold storage instead of cremation or re-burying it in an unnamed/fake name grave? So he and Hitomi could threaten Sejima at any time they wanted? Even when the plot tells me Manaka was their friend? What the hell is wrong with these people? 
Does it have any explanation or is it just a Japanese plot excuse?