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.
Marin does confess to Gojo in the manga. It happens during a crucial moment when their relationship reaches a turning point. The confession is filled with honesty and vulnerability.
Well, it depends on how you interpret the story. Technically, Gojo Satoru's fate is quite complex and can be seen as a sort of death in some respects, but there are also elements that leave room for interpretation.