Indeed, Koro Sensei passes away in the manga. This event is both emotional and crucial to the story's conclusion, leaving a lasting impression on readers.
I'm not very familiar with this light novel specifically. But generally, a light novel often features unique characters and a fictional story world. It might be about some kind of adventure or relationship in a setting where there's a'maou' (demon king) involved. Maybe it's about the interactions that take place on a certain 'floor' within this fictional universe.
Well, when referring to a manga writer, the term sensei is commonly used. It conveys respect and recognition of their professional status. Senpai is not the usual way to address them in this field.