Muichiro's death in the manga occurs during a particularly intense confrontation. His injuries accumulate, and he ultimately succumbs despite his valiant efforts.
Naruto's friends in Konoha would probably be a bit surprised at first. But they would soon come around and support him. His old mentor, like Iruka, would be happy for him. As for Mei's side in Kirigakure, her comrades might be a bit more cautious. But once they see how Naruto can contribute to their village as well, they would accept the relationship.
If RWBY were to react to Spy x Family fanfiction, they would bring their own perspectives. Ruby, with her innocent and curious nature, would likely be intrigued by Anya's telepathic abilities. Weiss, being more refined, might analyze the political and social aspects of the Spy x Family world. Blake, having experienced secrecy herself, would empathize with the characters hiding their true identities. Yang would enjoy the comical moments and the action - packed sequences, like when Loid does his spy stunts.
Gray would be extremely angry. He would immediately start looking for clues as to where Natsu was taken. His ice - make magic would be on full display as he might freeze things in frustration while trying to figure out the situation.
For Peter Parker's family, the news of his marriage to Scream would be quite a bombshell. Aunt May would likely question Peter's judgment. She might think that Peter is getting in over his head and that Scream is not the right person for him. But Peter would defend his relationship, saying that Scream has a good heart beneath her tough exterior. Regarding Scream's family, if they are part of a more villainous group, they might try to break up the marriage. But if they are more open - minded, they could see the potential in Peter and support the relationship. The reactions of their families would add a lot of drama and conflict to the fanfiction.
If the My Hero Academia characters were to react to Spy x Family fanfiction, it would be quite a spectacle. Momo Yaoyorozu could be impressed by the creative plots in the fanfiction. She might admire how the Spy x Family world is built with its own set of rules and characters. Kirishima would likely be excited by the action scenes. He'd be all about the times when Loid has to fight or use his spy skills. And Mineta? Well, he might have some inappropriate thoughts, but we'll just ignore that and focus on the more positive reactions of the other characters.