I'm not sure specifically what Ren Amaki's web novel is about as there could be many different themes and stories in it. It might be about adventure, fantasy, or some kind of personal growth story. Without more information, it's hard to say for certain.
I'm not sure where you can specifically read Ren Amaki's web novel. It might be available on some Japanese web novel platforms. You could try searching for it on popular Japanese literature websites.
The main distinction of porn with a story genre from regular porn is the presence of a coherent narrative. Regular porn often jumps straight to the sexual action, while the story genre tries to add some kind of story framework around it. But in the end, both are adult-oriented and feature explicit sexual content.
Well, it's hard to say for sure. The concept of 'porn-like' is quite subjective. Some might consider certain novels with excessive sexual descriptions as such, but it depends on individual perspectives and cultural standards.
In many places, 18+ porn comics are considered illegal. This is because they can have a negative impact on society and violate moral and legal standards.