edit; I guess we’re supposed to treat everything we see prior to chapter 14 as unreliable narrator that was edited later to add N to it? And really the only original N was the scribbling in chapter 1?
A lot of the early chapters are basically the "Fukushima version" of events, where unrelated stories he was told get some mysterious connection to N added to tie them together, and we see the "real" version of them in the back half of the story, like chapter 4 showing us The Tale of the High School Girl and the Haunted Karaoke Box, and then chapter 16 having Hiruma talking to the girl and learning how Fukushima heard the story from her while working as her tutor and embellishing it to give it more punch. You've also got chapter 5's The Tale of the Cute Omegle Girl and the Photo-eating Goldfish, and then chapter 18 shows us Fukushima hearing the original story in a restaurant and immediately making up a connection to the school massacre in chapter 2.
Some of the de-mythologizing starts before chapter 14, though. Chapter 3 shows us the NEET reporter hearing about N and going to investigate, where he meets a strange man, whose backstory we get in chapter 13: he's a socially-isolated, presumably mentally-ill child obsessed with biblical prophecy, with a particular fixation on a theoretical 14th apostle who is the
real culprit who betrayed Jesus, and who he refers to in his schizo-notebook as N because N is the 14th letter of the alphabet. Later on, as a teenager or young adult, he sees the beginnings of the media frenzy over N on TV and goes to investigate the mysterious shrine. Chapter 14.1 then shows him running into and ultimately murdering the NEET reporter there, and ending up in prison (also, his haircut in prison makes him look so much like Shindou that I totally red herring'd myself by thinking this was a flash-forward of Shindou in prison, but, no, it's a totally different dude). This same dude also appears in chapter 11 asking for directions to the bus stop in a police station before the police officer he talks to commits suicide, but according to chapter 14.1 it wasn't a suicide at all, and Crazy Dude actually murdered the policeman.
The plot's basically about how the original lie about N that the boy told to avoid getting into trouble after his friends drowned spirals out of control, helped along by Fukushima's click-baiting and people's innate desire to want an explanation for seemingly-inexplicable tragedies, which feed into each other and keep tying together unrelated incidents under the umbrella of N. And this innate desire for an explanation for the inexplicable will lead to this all happening again, with the kids showing up where it all started and one of them going "Whoa, what's with that creepy letter? It's like, a Z or something."
There's also a little bit of mystery left after it all, though. There are some genuinely unexplained paranormal occurrences throughout the story: Shindou's prophetic visions of people dying are the biggest one, but there's also Hiruma's recording getting mysteriously corrupted in chapter 17, his business card getting turned pitch-black in chapter 20, and his ghostly encounter that set him off down the path of a paranormal reporter in chapter 19. There's also the rather odd circumstances of Sanae's death, which may or may not have been just a tragic accident. We never get an explanation of any sort for these, but, well, when you stop and think about it all of these things have gotta be the work of Z, right?