Responses
Actually, Chiaki's story has been seperately elaborated in the author's novel set in the same universe as Sekaiichi Hatsukoi called Yoshino Chiaki no Baai (The Case od Chiaki Yoshino). I think the first few volumes are available in english in some website although I forgot which one since it's been years when I last read it.

okay so it’s been a while i didn’t read sekai
ichi so decided to reread it again but was the part about Yoshiyuki confessing to Chiaki while watching the fireworks never included in the manga ? I wonder if it’s only in the anime or my memories in playing with me ToT