Definitely not. The Avengers comics have a specific set of characters and Yondhu isn't part of that lineup. Maybe you're confusing Yondhu with someone else?
If Flash joined the Avengers in the Age of Ultron, his super - speed could have been a great asset. He could quickly reach danger zones, evacuate civilians much faster than anyone else. Maybe he could even run circles around Ultron's minions, confusing them and buying time for the Avengers to strategize.
It can also be frustrating at times. Some fanfictions lack the proper understanding of the movie's lore. They might introduce concepts that don't fit well with the established Marvel Cinematic Universe. But on the positive side, many fanfictions are a labor of love. Authors pour their passion for the movie into their writing, and that can really shine through, especially when they capture the essence of the characters as we know them from the movie.
Well, one reaction could be excitement. Fanfictions often explore different aspects of the characters and storylines that might not have been fully delved into in the movie. For example, they might focus on the relationships between the Avengers in more depth, like the budding romance between Black Widow and Hulk. It can be really interesting to see how different authors interpret the characters' emotions and motives.
Yes, it is. It gives more details about the story that the movie is based on. Fans can get a better understanding of the events leading up to the Age of Ultron.