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.
Yes, there are. Some movies draw inspiration from various sources but not directly from comic books. For example, 'The Truman Show' has elements of a fantastical world but isn't based on a comic.
Well, a blank comic book isn't really an actual comic book in the usual sense. It doesn't have pre-drawn characters, plot, or dialogue. It's more like a canvas waiting for your imagination to fill it up.