>>543549728
There's a few ways a lonely boy with absent parents, a grandma who might leave him too soon, a fear of being alone, and is afraid of losing his friends could find out about suicide and hanging. It's sad to say, but even young children can think about and commit suicide in the real world. It's not unreasonable to think Basil could have under his circumstances. In a situation like Mari's death, he's panicking and trying to think of a way for Sunny not to get blamed. It's still a bit crazy he decided to frame it as a suicide, but the pieces are there for him to know how.
As for how they could have hanged Mari, I'll just to defer to picrel.