Steve Rogers would probably be one of the most concerned. His sense of duty and protectiveness would make him take the 'get help' situation very seriously. He'd be worried about the safety of those in need.
Thor, in his own way, could be very concerned as well. He has a big heart and a sense of honor. If someone is asking for help, he would feel obligated to assist. He might not fully understand the modern - day nuances of the situation at first, but his concern would be genuine. He'd be ready to use his strength and powers to protect those in need, just like he does in Asgard.
The Avengers' reactions in this fanfiction would vary greatly. For example, Vision, being logical, would try to assess the situation based on data and facts. Wanda Maximoff might use her powers to sense any emotional distress related to this 'get help' situation. And Rhodey, if he's involved, would be practical, thinking about the best military - style approach to handle it. Each character brings their own unique perspective and skills to the table, which would make their reactions interesting to read about in the fanfiction.
They would probably be very surprised. Some might laugh at the creative ideas in the fanfiction.
I think Steve Rogers would be a likely ally. Steve is all about justice and doing what's right, and Castiel has a similar sense of duty in his own way. They could bond over their strong moral compasses.
In this fanfiction, some Avengers might be in shock. For example, Captain America could be deeply saddened, his belief in justice and protecting the world shaken. He might feel a sense of failure for not being able to prevent the Snap.
I think it could be Clint. He has a family and the thought of his family being Snapped away would be devastating. He would be haunted by the idea that he wasn't there to protect them. He might feel a great sense of guilt and loss compared to the others.
Iron Man. They both are very intelligent and rely on their technology a great deal. They could share ideas about improving their gadgets and suits.
Iron Man would probably be the most interested. He's always into new technology and Spider - Man's web - shooters would be something he'd love to study and maybe even improve.
I think Thor would enjoy it a great deal. He loves new experiences and the grandeur of Disney World would appeal to him. The big parades and shows would seem like great feasts to his eyes.
Iron Man would be angry. He'd probably use his technology to try and track Percy down. He might also start to review all the security protocols to make sure no other traitors could do the same. Thor would be more of the 'let's fight him head - on' type, but the other Avengers would have to hold him back until they have a proper strategy.
The Avengers would initially be in shock when they find out Peter is sick. Clint Barton would be the one to keep a calm head and start organizing things. Wanda Maximoff would try to use her powers to ease Peter's pain if possible. Vision would use his vast knowledge to try and find a solution. They would all be very concerned and do everything they can to make Peter better, from getting the best medical help to providing a safe and comfortable environment for him to recover.