In such a fanfic, Snape might initially be shocked. Given his complex relationship with Harry in the original series, he could struggle between his old grudges and his sense of duty. He might take it upon himself to protect Harry secretly at first, perhaps by using his position at Hogwarts to keep an eye on Harry's abusers. For example, if it's the Dursleys abusing Harry, Snape could find ways to make sure they don't go too far without revealing his true intentions.
If Snape finds Harry Potter fanfiction, he'd probably be quite irritated. He has a rather stern and no - nonsense personality. He might see it as a frivolous waste of time and be disdainful of the creative liberties often taken in fanfiction. He'd likely scoff at the idea of people writing about his students and the wizarding world in such unregulated ways.
In the fanfiction world, if Snape finds Harry cutting, he might be initially shocked and angry. He could see it as a sign of Harry's inner turmoil. Snape, despite his complex relationship with Harry, might feel a sense of responsibility. He might immediately take steps to stop Harry from further harming himself, perhaps using his magic to heal the cuts. And then he would likely question Harry about why he was doing such a thing, trying to get to the root of the problem.
Well, first of all, the students would probably be shocked. His friends, like Ron and Hermione, would be extremely angry at the Dursleys. They might try to comfort Harry more often. And the school itself could start providing special resources for Harry, such as counseling or extra protection spells. Also, there could be a big change in the way the wizarding community views Harry. Instead of just seeing him as the 'Boy Who Lived', they would see him as a victim who needs support.
Well, if we consider the 'umbridge finds out harry was abused fanfiction', it really depends on the writer's take. One possibility is that Umbridge, being the power - hungry character she is, might try to cover it up to maintain the status quo at Hogwarts. On the other hand, she could also see it as a way to discredit Dumbledore, thinking he failed to protect Harry. There's also a chance that the discovery could lead to a change in Umbridge's relationship with Harry, from being a tormentor to an unexpected ally, though this would be a huge shift in her personality.
One place to look could be Archive of Our Own (AO3). It has a vast collection of fanfics, and you can search for ones with this specific plot. Just use the search bar and type in relevant keywords like 'Snape finds abused Harry'. Another option is Fanfiction.net, though the tagging system there might require a bit more digging to find exactly what you want.
In the fanfic, if Harry Potter finds Mjolnir, he could use it to defend Hogwarts in a very different way. Instead of using magic spells, he could use the power of Mjolnir to smash through dark magic defenses. And his friends would be both amazed and a bit worried about this new power he has acquired.
Well, some Slytherins might initially be skeptical. They are known for their cunning and might think it's a ploy. But if the evidence in the fanfic is convincing, some could feel a strange sense of sympathy. After all, they are not all bad. They might start to see Harry in a new light, not just as the Gryffindor hero but as a victim too.
In this kind of fanfiction, there could be a lot of shock and disbelief among the characters at first. His friends might feel guilty for not noticing the signs earlier. They would probably become extremely protective of Percy.
In a fanfic where Snape teaches Harry properly, Harry might become an even more powerful wizard. Snape's deep knowledge of magic could be fully imparted to Harry. Harry could learn advanced potions techniques more quickly and might also gain a better understanding of the Dark Arts not for evil but for defense. This could change the whole dynamic of his battles against Voldemort as he would have a more comprehensive magical skill set.
Snape's reaction could be quite mixed. On one hand, he might be angry at Dumbledore for not doing more if he knew about the abuse. He may also feel a strange sort of empathy towards Harry. After all, Snape himself had a difficult childhood. So, he could start to be a bit more lenient with Harry in his classes and try to subtly offer support.