Harry might initially be in shock. He could feel a mix of anger and helplessness. Maybe he'd try to hold back his magic out of fear of getting into more trouble with Vernon.
Vernon might be jealous of Harry's magical abilities. He could see Harry as a threat to his 'normal' family life and his authority in the household.
In some fanfictions, Harry might cope by finding solace in his friends at Hogwarts. He could talk to Hermione and Ron about what Vernon does, and their support gives him strength. For example, they might encourage him to focus on his magic studies as an escape from the Dursley's household.
Well, perhaps Vernon might beat Harry out of his extreme dislike for magic. He could be enraged by some accidental magic Harry does at home and lose his temper completely.
It might be a fan who was really intrigued by the relationship between Harry and Vernon and decided to take it to an extreme in their writing. There are so many fanfiction writers out there, some write for fun, some to explore different character dynamics. So, it could be anyone really.
The wizarding world would likely be in shock. Harry is seen as a symbol of hope against Voldemort. His death at the hands of Vernon would be a huge blow. There would probably be a lot of anger towards the non - magical world for allowing such a thing to happen.
Snape might initially be shocked. He has always had a complex relationship with Harry. He could be hesitant at first, not fully believing Harry, but then as the story progresses, his protective instincts might kick in and he could start to investigate the abuse further.
Severus might initially be shocked. He could then feel a sense of protectiveness over Harry. He might start to secretly look out for Harry, like making sure he has enough to eat at Hogwarts and is safe from further harm.
Harry might cope by retreating into his own thoughts and memories of his parents and the wizarding world. For example, he could think about the good times at Hogwarts to endure Vernon's torture.
Perhaps Dudley finally stands up to his father's cruel treatment of Harry. Maybe he realizes that Vernon's behavior towards Harry is wrong and uses his own physical strength or some clever ruse to get Harry out of a difficult situation Vernon has put him in.
He could be in shock at first. Just standing there, unable to believe what has happened.