In some versions of the fanfic, Harry might have a sort of instinctive understanding that his parents had a good reason for leaving him there. So, while he's still not happy about it, he might be a bit more accepting. Maybe he can sense some sort of protection spell around the Dursleys' house that gives him a small sense of security.
Lily would probably be very warm and welcoming. She was known for her kindness. She might immediately take a liking to Ginny, seeing her as a strong - willed and intelligent young witch, much like herself. James, on the other hand, might be a bit more boisterous in his welcome. He could be impressed by Ginny's skills in Quidditch, if that comes up in the meeting, and would likely be glad that Harry has found such a remarkable girl.
In some fanfictions, Harry could react in a very calculated way. He might start to plot a way to get back at Aunt Marge without using magic in an obvious way. He'd use his wits and the knowledge he has of her weaknesses. For example, if he knows she's very proud of her dogs, he could find a way to make her look foolish in front of others when it comes to her precious pets. All this while still seething about the insults to his parents.
In some fanfictions, magic might be used to bring them back. For example, a powerful time - turning spell could be involved, allowing them to be retrieved from the past before their deaths. Another way could be through some sort of deal with dark forces, although this would likely have many negative consequences.
Yes, there might be. Some fanfics like to subvert the expected negative situation. In these fanfics, even though the start is that Harry's parents hate him, through a series of events such as Harry showing extraordinary courage or kindness, his parents could have a change of heart. For example, he might save them from a great danger, which makes them realize their mistake and start to love him. This kind of plot twist can add more depth and surprise to the fanfic.
In these fanfics, a common theme is growth. Harry has to grow up faster and become more independent because of his parents' hatred. Another theme is the search for identity. Without the positive influence of his parents, he has to define himself. And the theme of resilience is also prominent. Harry has to be strong in the face of his parents' negative feelings and all the difficulties that come with it. He has to keep going and not let their hatred break him.
In some fanfics, they might abandon Harry because they are under a dark spell. This spell could make them lose their love for him and force them to leave. Another reason could be that they are blackmailed by some powerful dark forces. These forces may threaten their lives if they don't abandon Harry.
Harry would probably be overjoyed at first. After all, he has always longed for his parents. He might start to look for ways to make it happen, like searching for magic spells or ancient artifacts that could bring his parents back.
At the start, Harry would be stunned. His whole life has been built on the idea of his parents being heroes. When he realizes they betrayed him, he would be heartbroken. He might go on a journey to find out the real motives behind their actions. Along the way, he could become bitter and vengeful. However, he might also find some inner strength and decide to rise above the betrayal. He could use it as a motivation to break free from the shadows of his parents and create his own path, but not without a great deal of emotional turmoil first.