
couldn’t the old man have seen jinu when he cleaned the pigpen? or anyone who disposed of the pig for that matter? it’s so far-fetched that they just forgot to feed/clean the pigs after jinu disappeared

From one of the scenes, it was that Jinu was under the platform the pigs were roaming around. But when he was retrieved; it's shown that he was buried outside the Pigpen. So, I'm not quite sure.
Regardless of whatever it is, it would've been pretty difficult for the family to figure it out. It wasn't a crowded place and they weren't people who know the brutality of human world by watching such news or crime documentaries. It's incomprehensible even for someone addicted to crime shows that a child could dig and bury another child.
However, the police had a great chance to notice as the soil or platform would be different. Since they searched the forest, they should've searched the house too.
im so happy to find another manga with similar vibes as chibi maruko chan yet its also very original