She might be heartbroken. Hermione is a strong - willed character, but being cheated on would surely shatter her. She could initially be in disbelief, given her trust in Ron. Then, she might become angry and start to distance herself from Ron, focusing more on her own studies or other relationships, like her friendship with Sirius.
Sirius could play multiple roles. Firstly, he could be a friend who listens to Hermione's rants and sorrows. Secondly, he might act as a protector, making sure Hermione doesn't do anything rash in her emotional state. And thirdly, he could be a catalyst for Hermione's growth. His presence could make Hermione realize that there are other aspects of life and relationships that she has been overlooking. His friendship could open up new opportunities for her, whether it's in terms of magical knowledge or personal development, all while she is dealing with the aftermath of Ron's cheating.
Hermione would probably be extremely hurt. She's a very loyal person, and to have Ron cheat on her would shatter her trust. She might become withdrawn, not wanting to talk to anyone at first, and just try to process the pain on her own. Then she decides to run to Lucius, perhaps because she feels she has nowhere else to turn, or she thinks Lucius can offer her some sort of solution or support.
She might be extremely hurt and angry. In such a situation, running to Sirius could be her way of seeking comfort and support from someone she trusts. She could be feeling betrayed and might have a hard time believing that Ron would do such a thing.
There could be. Fanfiction is a vast and diverse world where authors can take the characters in all kinds of directions. But it's important to note that this isn't in line with the general perception of the characters' relationships in the official books. It might be an interesting exploration for some who like to see how different twists can change the story, but others might be put off by it.
Perhaps she tries to find them right away. Using her intelligence and resourcefulness, she starts to search for any clues as to where they might have gone. She could ask other friends, like Neville or Luna, if they have seen Harry and Ron. And she might be determined to bring them back, not willing to give up on their friendship easily.
In many fanfics, their relationship develops through shared hardships during the search for Horcruxes. They rely on each other more and more, which deepens their emotional connection.
He could also be heartbroken. Ron might withdraw into himself, spending a lot of time alone in his room at the Burrow, not wanting to talk to anyone. He may start to doubt his self - worth, thinking that he wasn't good enough for Hermione.
I'm sure there are. Fanfiction is a vast world where all kinds of scenarios are explored. A 'Ron cheats on Hermione' fanfic might be written to show the vulnerability of their relationship or to introduce new plotlines. But it's not something that would be acceptable in the context of the original story. It can be an interesting read for those who like to see different interpretations of the characters, but it also might be a bit controversial among die - hard fans of the original relationship.
Hermione would likely be very angry at first. She is a strong - willed character and being kidnapped would go against all her principles of freedom and rights. She would probably try to reason with Ron immediately, using her logical arguments to make him see how wrong his actions are.
Another way she could react is by distancing herself completely. She would stop trying to be part of their group and start creating her own identity at Hogwarts. She might explore parts of the castle she's never been to before, find new study nooks, and interact with students she's never really talked to, all while trying to forget about Ron and Harry's cold treatment.