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.
He would probably be quite amused by some of his past antics. For example, his constant pranks on Iruka-sensei. But at the same time, he'd also see the pain he carried from not having his parents and realize how that influenced his actions. He might react with a bit of regret for some of the stupid things he did out of loneliness, but also be glad that those experiences made him who he is today.
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.