Well, first of all, Snape is a very complex character. If he found out about the DA fanfiction, he would most likely be offended. He has a strong sense of order and propriety at Hogwarts. The DA was a group that defied the school's authority, at least in the eyes of the strict professors. Fanfiction, which often takes creative liberties, would seem to him like a misrepresentation of the events and people involved. He might feel that it was disrespectful to the school's traditions and his own position as a teacher. Moreover, he might be worried that it could inspire more students to engage in rebellious behavior, so he would surely take some sort of action to deal with it.
Snape might be furious. Given his personality, he is not one to tolerate any sort of what he would consider rule - breaking or inappropriate glorification. The DA was a thorn in his side during the school year, and fanfiction about it would be like rubbing salt in the wound. He'd likely want to put a stop to it immediately and punish those involved if he could.
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.
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.
Well, Percy has seen a lot in his adventures. At first, he might be a bit taken aback because fanfiction is a whole new world compared to his real experiences. However, as he realizes that it's just fans exploring different scenarios with the characters, he could become rather indifferent. He might even start to see the value in it, as it shows how much the fans care about the overall story and its characters. He might think it's a little strange that people are so interested in writing about Nico and Will's relationship, but he'd respect the creativity of the fans.
There would be chaos. Snape is not one to let things slide easily. He'd probably start investigations, interrogate students, and this would disrupt the normal school life. The students involved in the DA or the fanfiction might have to find ways to cover up or face Snape's wrath.
He might be quite shocked at first. After all, Wolfstar involves his dad's best friend Sirius and his other dad figure Lupin. It's a rather strange concept for him to think about them in a romantic way as he knew them in a different context.
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.
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.
Cedric would be deeply troubled. In the wizarding world, they have their own code of honor and respect. Finding out about Harry's abused fanfiction would go against everything Cedric stands for. He might start to question the morality of the Muggle world that allows such things to be written. He would also feel a sense of sadness for Harry, knowing that someone out there has created a story that portrays him in such a negative and harmful light. Cedric might even consider taking some sort of action, like talking to Harry about it or trying to get the fanfiction removed if he could, though that would be a complex matter given the nature of fanfiction and the different worlds they live in.
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.
She might be quite surprised at first. Fanfiction often takes the relationships of the characters in unexpected directions. Molly is a rather traditional character, so she could be a bit taken aback by some of the more creative interpretations in the fanfiction about Ron and Hermione.