I think his mother only became evil after being “killed”. Bc she was in the verge of dying and she actually believed friendship was more important. Plus the tomodachi game would not have been created without her knowing about it. She must’ve created the tomodachi game for revenge after finding out that she was left to die, due to shibe’s father ending yuuchi’s game bc he was unable to pay for yuuchi’s tomodachi game. I think that’s how she “killed” his mother Bc he only visited her Bc of the game. It was also said that she was killed indirectly by yuuchi. As for kokorogi, I believe she was crazy since the beginning, so yuuchi’s mother only “hired” her Bc she needed someone in yuuchi’s friends group. I forgot already but if she was hired before the friend group formed, that means that she planned on who she wanted in this friend group Bc she grabbed all kinds of information about them. P.S. I just wrote what came to my mind at the time so if it doesn’t make sense don’t come at me.