Snape's reaction could be quite intense. He might first isolate Harry to keep him safe from further harm or rumors. Then, he would likely dig deep into the matter, using his potions skills to perhaps create something that could help Harry heal from the mental and physical scars. His actions would be driven by a new - found respect for Harry's vulnerability and a desire to right the wrong done to him.
In the fanfiction world, Snape might be initially shocked. He has a complex relationship with Harry. He could feel a sense of protectiveness that he rarely shows openly. He might use his skills and knowledge to find the perpetrator and seek justice for Harry.
Snape would probably be very conflicted. On one hand, he has his grudges against Harry's father. But on the other, his sense of right and wrong would be stirred. He might take it upon himself to train Harry to be stronger, both physically and mentally, to overcome this trauma.
Snape would be appalled. He has a deep connection to the world of Harry Potter, and the idea of a rape romance fanfiction is so far removed from the values of the wizarding world he knows. He might initially be in disbelief, thinking it was some sort of dark joke or a misunderstanding. But once he realizes it's real, he would likely feel a mix of anger towards the creator and sadness that such a thing exists in relation to a world he cares about.
He would probably be furious. The blood quill was a very cruel and unjust tool used on him as a student. Discovering fanfiction about it might make him feel that his past pain is being exploited or made light of.
Snape's view of Harry might become more sympathetic. He would see Harry as a victim rather than just an annoying student. This could lead to a softening of his attitude towards Harry in general.
In some fanfictions, Snape might be secretly sympathetic. He may see the scars as a reminder of all Harry has been through, which could soften his usually harsh attitude towards Harry, at least for a moment.
Harry's reaction would be complex. He has so many emotions tied up with Snape already - dislike, confusion, and a bit of grudging respect in some ways. Discovering he's his father would bring all those emotions to the surface and more. He might go through a period of denial, not wanting to believe it at first. Then, as the truth sank in, he'd have to come to terms with a new family member. He might also feel a sense of betrayal from those who knew or suspected but didn't tell him. And he'd have to figure out how to fit this new relationship into his life, especially considering his role in the wizarding world and his ongoing battles.
He would probably be very displeased. Snape has always been strict and traditional, and fanfiction about the DA, which was an unauthorized student group in his eyes, would seem like a frivolous and inappropriate thing to him.
Well, Sirius would have a complex reaction. On one hand, he was very close to James, and his brother was Regulus. The idea of them in a romantic relationship in fanfiction would be hard for him to wrap his head around. He might start to question how people could come up with such a thing. He might also be a bit protective of James' memory, if he thinks the fanfiction is portraying James in a way he doesn't like. However, he might also be secretly a bit intrigued, because it's such a different take on the relationships he knew, and it might make him think about James and Regulus in a new light, but he'd never admit it out loud.
Snape's initial reaction would be one of disbelief. He has his own set of prejudices and ideas, and gay romance fanfiction is far from his realm of understanding. He might snatch the fanfiction away from Harry in a fit of annoyance. But as he sees Harry crying, he might start to wonder what in this strange piece of writing has such an emotional impact on Harry. This could lead to a very awkward and tense moment between them, with Snape not knowing how to handle the situation properly.
Snape would be deeply shocked and disgusted. Given his own experiences and the values he holds, finding Harry in a rape - related romance fanfiction would be like a blow to his perception of the world around Harry. He may initially be in disbelief, but then his anger would rise. He might try to find out who created such a disturbing fanfiction and perhaps try to put an end to it, as it goes against everything he believes in and the world he is a part of in the original Harry Potter series.
Dumbledore's reaction would be very complex. He was always aware of many secrets. If he knew about this, he might have tried to keep it hidden for some greater purpose. But when it was revealed, he would likely have a look of both regret and understanding. He might regret not having told Snape or Harry, but also understand why it was a difficult situation to disclose. As for Voldemort, he would probably see it as a weakness in Snape and try to use it against him in some way.