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.
The title of the text was seven matches because this article told the story of a person who matched matches in winter. In this article, matches were not only a lighting tool but also represented warmth, hope and courage. Through this article, readers can feel the power of the match and also feel the warmth and hope of winter. The role of the seven matches is to provide readers with a profound revelation to let the readers feel the preciousness and beauty of life.
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.