Well, in such a fanfiction, Draco might be completely out of his element when he meets the Dursleys. He could be shocked by their rudeness and lack of magic knowledge. Maybe he'd initially look down on them but then gradually realize there's more to them than meets the eye.
One interesting plot point is that Draco, being a Slytherin, might try to use the Dursleys for his own gain at first. But as he gets to know them better, he realizes that they are more complex than he thought. Also, the cultural clash between the wizarding world Draco comes from and the Muggle world of the Dursleys can lead to many humorous and thought - provoking situations. For example, Draco's confusion over Muggle appliances and the Dursleys' bewilderment at Draco's magic.
They would probably be horrified. The Dursleys have always despised magic and those associated with it. Teddy, being a young wizard, would represent everything they dislike. They might try to shoo him away or call the police, thinking he's some sort of freak.
The wizarding world would likely be in shock. Harry was seen as a hero, and this act would completely change their perception of him. Some might think he was finally getting revenge for all the abuse he endured, but many would condemn his actions as murder is still a serious crime in the wizarding world.
Percy would likely be very critical of their lifestyle. He comes from a world where family is important and they support each other in the face of danger. The Dursleys, on the other hand, seem to shun anything out of the ordinary and are unkind to Harry. Percy might try to show them a different way of living, with more love and acceptance, though they probably wouldn't listen.
They would likely be disgusted. The Dursleys are so anti - magic that any form of fan - created stories about the magical world would be abhorrent to them.
The Dursleys' reaction would be complex. Vernon would be enraged and might even try to burn any fanfiction he found. Petunia would be conflicted. Part of her would be disgusted, but a small part might be a bit curious as she has some connection to the wizarding world through Lily. Dudley might be more interested than his parents, especially if the fanfiction had some action or adventure that appealed to his more adventurous side.
In a fanfiction, Sirius' reaction to catching Drarry could be complex. He has a deep bond with Harry, so his first instinct would be to protect Harry. If he caught them, he'd want to know what was going on. He might interrogate them separately. He could be especially tough on Draco, given his family's history. But if the story has developed Draco's character in a positive way, Sirius could eventually come around and see that their relationship might not be as bad as he initially thought. It all depends on the specific plot and character development in the fanfiction.
They would be devastated. Harry has many friends and allies. People like Hermione and Ron would be heartbroken and would probably seek revenge. The Hogwarts faculty would also be deeply affected. Dumbledore, if he's still alive in the fanfiction, would feel a great sense of guilt for not protecting Harry better. There might be a split in the wizarding community, some believing the Dursleys were acting on their own, others suspecting a darker conspiracy.
The Dursleys would most likely be horrified. They have a deep - seated hatred and fear of magic, and Ron and Hermione are clear representatives of the magical world. Vernon might turn red in the face and start shouting, while Petunia would clutch her pearls and Dudley would cower behind them.