If the Volturi read Twilight fanfiction, their reaction could be complex. They are a proud and powerful group. Some fanfictions might show them in a less than flattering light, which would surely annoy them. Also, fanfiction often contains ideas that are not in line with their strict vampire code. They may initially be curious, but as they read more, they could become increasingly frustrated and perhaps even take action to suppress what they see as false or inappropriate interpretations of their world.
They would probably be very critical. The Volturi are all about maintaining the status quo in the vampire world as they see it in the Twilight series.
The Cullen family members will have different reactions. Carlisle, as the patriarch, would be diplomatic. He would try to smooth things over and show that the Cullens are following the rules. Alice might be using her gift to foresee any potential danger during the visit. Emmett, on the other hand, would be more ready for a fight if it came to that. As for the Volturi, they would come in with an air of authority, and their members like Aro would be observing every little detail of the Cullens.
They are often very possessive. Since they are powerful and used to getting what they want, once they find their mate, they will do everything to keep the reader close. For example, Aro might be very intrigued and want to study the reader if they have something unique, while Caius could be more aggressive in claiming the reader as his.
They might be either amused or offended. The Volturi are very traditional and strict in the Twilight universe. Fanfiction often takes creative liberties that could go against their values.
They might be either amused or offended. Fanfiction often takes creative liberties with the original characters and storylines. If the Volturi in the fanfiction are depicted in a way that goes against their self - perceived dignity in the original Twilight Saga, they could be offended. On the other hand, they might also find some of the more creative interpretations and new plotlines quite amusing.
They might be indifferent. After all, they have more important vampire - related matters to deal with. Bella's interest in Greek god fanfiction may be just a minor and strange human hobby to them.
Edward would be completely devastated. He would likely go into a blind rage and seek out whoever was responsible for Bella's death. His love for her was so intense that he would stop at nothing to avenge her.
The Twilight characters would be quite shocked. Vampires like Edward might be very suspicious of Hermione's new status as an imprinted character. Werewolves like Jacob would have a different reaction. Since imprinting is a big part of their culture, they might be more accepting but also cautious as Hermione is an outsider from a different fictional universe. They would need to figure out how to incorporate her into their pack or deal with her in some way.
Probably not. The Volturi are used to being in control and having a certain level of respect. Fanfiction can be quite wild and unpredictable, with characters acting out of character as they are in the original Twilight Saga. This lack of order and respect for their established selves would likely make them dislike reading fanfiction.
Jacob, in his werewolf form, would be more likely to directly confront Jason. He's used to fighting against threats, and his strength and speed as a werewolf would make him a formidable opponent. However, he might be a bit overconfident at first, not fully realizing the unique danger that Jason represents.
One common theme is power struggle. Since the Volturi are so powerful, fanfictions often explore how Bella either challenges or is influenced by that power. For example, she might try to assert her own will against their rules.