It starts at a specific point in the manga, but I can't recall exactly without looking it up. You might want to check some manga forums or dedicated fan sites for the precise chapter.
One possible plot could be that in a sparring session, Himawari, who has been training secretly, surprises Naruto with her newfound strength and accidentally knocks him out. This could lead to a series of events where the village is in a bit of chaos as they wonder how a young girl managed to take down the Hokage. Then, Himawari feels guilty and tries to make it up by training even harder to protect the village in the future.
Sukuna does die in the manga. The way it happens is quite dramatic and ties in with many other elements of the story, adding depth and tension to the overall narrative.
Gomi Gojo's Territory Opening Gesture was an important element in the " Return of Curse ". Gotto Gojo's domain was displayed as 'Boundless Empty'. This gesture was to pull the enemy into the infinite inner side, forcing the enemy's brain to produce a large amount of information, causing the brain to lose its function and could only stand and wait for death. Although there was no specific description of the hand gesture, it could be speculated that the hand gesture was to expand the domain through a specific posture of the hands.
Sakura would probably be shocked and immediately rush to Naruto's side to check his condition. She might also be a bit angry at Himawari at first, but then realize it was an accident. Boruto would be in a state of disbelief and would try to comfort his sister who is likely feeling guilty.
No. Ryomen Sukuna is a fictional character from Japanese mythology and modern manga/anime. He doesn't have a basis in a true historical event or person.