Kageyama stays with Karasuno in the manga. His character development and contributions to the team continue without him leaving. The bond he forms with his teammates keeps him committed to the club.
Sure, Karasuno beats Shiratorizawa in the manga. The match is full of thrilling moments and showcases the growth and determination of the Karasuno team.
They might be really surprised. Some characters could be amused by the creative scenarios fans come up with. For example, Hinata might be excited to see himself in all kinds of wild adventures in the fanfictions.
The characters might be a bit shy. For instance, if there are fanfictions about Tsukishima and Yamaguchi's friendship, Tsukishima might act like he doesn't care but secretly be a little flattered. Yamaguchi, on the other hand, would probably be more openly happy about it.
I have no definite answer. Sometimes manga series end when the story feels complete to the creator or when publishing circumstances change. So, we'll just have to wait and see.
No, there isn't. The main emphasis of Haikyuu is on teamwork, friendship, and the pursuit of excellence in volleyball. Love stories are not a significant part of the plot.