Who usually punishes the Dursleys in 'the dursleys get punished fanfiction'?

One scenario could be that Dudley gets turned into a frog. It would be a fitting punishment for his years of bullying Harry. Another could be that the Dursleys' house starts to have strange magical malfunctions, like the stairs turning into a slide, constantly tripping them up.

Yes, in many fanfictions, the Dursleys face consequences for their mistreatment of Harry.

The Dursleys would probably be more frightened. They are muggles and Snape is a powerful wizard. The idea of a strange man in black robes suddenly appearing and using magic would be terrifying for them.

It's possible. In some fanfictions, authors might explore complex motives for the Dursleys' actions. Maybe they could be under some dark magic influence that makes them act out of character and in a more extreme way. However, this goes against the general perception of the Dursleys in the original series, where they are mainly just cruel and neglectful without such extreme murderous intent.

In fanfiction, the Dursleys could get arrested for child abuse. They treated Harry Potter horribly, locking him in a cupboard, underfeeding him, and constantly berating him. Another reason might be for illegal use of magic items. If they got hold of some magical objects through Harry and misused them, that could lead to their arrest.

Well, Daphne in the 'Daphne meets the Dursleys fanfiction' would probably interact with the Dursleys in a very cautious way at the start. She would be observing their behavior towards Harry closely. Once she realizes the extent of their nastiness, she could try different approaches. She might try to reason with them, explaining how wrong it is to treat a child like that. When that doesn't work, she could use her magical abilities to disrupt their daily routine in a harmless but annoying way, like making the TV constantly change channels or the kettle never boil. This would be her way of getting back at them for their treatment of Harry and also a way to show them that there are forces they don't understand.

He might be very angry with them at first for how they've treated Harry.

Remus might interact with the Dursleys in a very calm and collected manner. He would probably be polite, despite their rudeness.

Well, the main characters are usually James Potter, Sirius Black, Remus Lupin, and Peter Pettigrew. They are the marauders after all. When they get punished, it's often centered around their misadventures together.

The Dursleys are a rather uptight and narrow - minded family. In this fanfiction scenario, they could be extremely judgmental of Harry's girlfriend. They might look down on her if she doesn't fit their idea of what a proper person should be like. They may also be secretly afraid that she will expose their mistreatment of Harry, so they could act very defensively during her visit.