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.
In the 'Daphne meets the Dursleys fanfiction', it's possible that Daphne initially has a hard time understanding the Muggle - like ways of the Dursleys. However, as time passes, she might start to see through their facade. She could then form a plan to get Harry out of that unpleasant environment. For example, she could pretend to be a social worker or something similar in the Muggle world and convince the Dursleys to let Harry go with her. And then she could introduce Harry to the wizarding world in a more positive and gentle way than how the Dursleys have been treating him.
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.
Snape would likely be disdainful towards the Dursleys. He has little patience for Muggles in general, and the Dursleys' mean - spiritedness would not endear them to him. He might use his cold, sarcastic tone on them, leaving them confused and a bit frightened.
Daphne could be a fictional character created specifically for this fanfic. She might be a witch from a well - known wizarding family. Maybe she's a classmate of Harry's at Hogwarts or someone who has just heard about his situation and feels compelled to help.
The Marauders would likely have a very negative reaction to the Dursleys. Sirius, having run away from his own awful family, would not tolerate the Dursleys' treatment of Harry. He'd probably confront them right away.
Ginny might be very curious about the Dursleys at first. But once she realizes how mean they are to Harry, she would likely be confrontational. She may not tolerate their rudeness and might even hex them a little if they push her too far.
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.
He might interact with suspicion at first. Given what he knows about how they treated his father, he'd be on guard. But then he could be polite just to see how they respond.
It could be that Daphne has a personal connection to Harry that she's not aware of until she starts investigating the strange magical disturbances around the Dursleys' place. Once she realizes it's Harry in trouble, she enlists the help of her friends or some sympathetic wizards to break Harry free. She might also face some challenges along the way, like dealing with the Dursleys' resistance or the Ministry of Magic's rules about interfering in Muggle affairs.