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.
If Snape never knew Harry was his son in this fanfic, it would completely change the dynamic between them. Snape's constant animosity towards Harry in the original series was partly due to his love - hate relationship with James Potter. But if Harry was his son, his actions would be more conflicted. He might still be harsh on Harry to hide his true feelings, but there would be moments of involuntary protectiveness.
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.
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 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.
Regarding the other teachers at Hogwarts, some might be more lenient with him because of Snape, while others might be more strict, trying to make sure he doesn't get any special treatment. His relationship with Neville could also be unique. Neville, who had his own experiences with Snape, might initially be wary of the son. But as they get to know each other, they could find common ground in their struggles and in their attempts to break free from the shadows of their fathers' reputations.
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.
Dumbledore would be a key figure in the reactions. If he believed in Snape's innocence, he would try to find a way to free Snape while also not alienating Harry. He might start his own investigation to uncover the truth. The other Hogwarts teachers would be in shock. Some might believe Harry is right, especially those who have always been a bit suspicious of Snape, while others would be outraged and demand an explanation from Harry.
Well, this is a really tough situation. The son could initially feel a sense of betrayal. His view of his mother would be completely shattered. He might also worry about how this could affect the family's reputation. He would likely need a lot of time to process his feelings and figure out how to deal with this new knowledge.
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.
Some Marvel characters would be really curious. Iron Man, for instance, would probably want to study Harry's magic, see if he could incorporate it into his technology. Thor might be a bit confused at first but would likely respect Harry's power as a wizard. And Captain America would see Harry as an ally right away, given his sense of justice and heroism. Harry's magic would be something completely new to them, and they'd have to figure out how to work with it or against it, depending on the situation.