Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
Most likely not. Many such stories are made up to evoke certain emotions or tell a compelling tale rather than being based on real events. Sister Death might fall into this category.
I'm not sure. It could be a real story that's been fictionalized for dramatic effect, or it could be entirely made up. You'd have to do more research to know for sure.