It gives fans a chance to explore different 'what - if' scenarios. By seeing the Avengers' reactions, fans can get a deeper understanding of the characters.
He might also get angry. Peter has a strong sense of justice. If he feels that the Avengers are being unfair, he could stand up to them. He'll use his quick wit and his powers to show that he's not going to be pushed around easily.
Some of them would be amused. Hawkeye might chuckle at the outlandish scenarios in the fanfic. Black Widow would be more reserved but still interested. She'd be quietly observing and thinking about the implications of the story. Captain America would try to find the moral of the story, if there is one, as he always does.
One interesting idea could be the Avengers' reactions to seeing alternate versions of themselves. It adds a new layer of self - discovery and character exploration. For example, they might see a version where they made different choices and learn from it.
Some of them could be in denial at first. They might think it was some sort of trick or false vision. But as they watched more and more of Infinity War, they'd have to accept the reality. Then, they'd start to think about how to use this knowledge. Maybe they'd try to find a way to go back in time like Dr. Strange did in the real movie, or find a weakness in Thanos that they didn't notice before.
Some of the Avengers would be very protective. Captain America would see Harry as a kid in need of guidance and safety, just like he does with his teammates. Tony Stark might be initially skeptical but would soon see the potential in Harry's magic. Thor would probably be fascinated by Harry's magical powers and see him as a worthy addition to their group.
Some of them might be entertained. For example, Thor could see the similarities and differences between his world and Percy's world of gods and monsters and find it humorous. Others like Black Widow might analyze the characters' motives and strategies in the fanfic as she is used to understanding people's minds.