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.
Well, it really depends on the specific fanfiction. Some fanfictions do make Ichigo overpowered. They might do this to explore different storylines or just for the fun of it. For example, in some, he could have a sudden boost in his reiatsu that makes him almost unbeatable. But not all fanfictions take this route. There are also those that focus on his character development in a more realistic way within the context of the Bleach universe and don't make him overly powerful.
It could be a sword that can cut through any substance, like the Susanoo easily. Just imagine Naruto swinging it and slicing through powerful jutsu defenses as if they were paper.
It's hard to pinpoint exactly where Gojo is at any given time in the manga. His whereabouts change as the story progresses and different scenarios unfold. It could be in a supernatural realm, a mission site, or interacting with other characters.