According to the +18 version, Yuri is the main heroine.
Also, couldn't Sou have kept walking past Yuri and walked around the back of the room to get to the protagonist seat? He didn't need to take that long route to get there.
Agreed. In the original he was a harmless perv, and mostly just to the ghost girls.
It also seems like the ghost girl is more of an afterthought than the main focus of the story.