I think that when a story would be considered non fiction would be that when you can relate to the story so well that you can even have a story of your own of what would happen because when you can relate to it that easily you could possibly think of yourself being in the story and living it out and if its so relatable be able to predict what could happen next or be shocked what could happen next.
Yes have truths are okay , because Laila Lalami a writer quotes "When your writing non fiction to me its based on fact, naturally in the process of writing there are going to be things you remember differently than other people" and she talks about when people write about a personal experience they are going to remember it by their memory and personal experience, and when James Frey does talk about the accident that occurred between the 2 students he did lie on that he caused it but he most likely wrote on how he felt when it happened
No, mostly because though when we read a novel we don't really care what genre it would be if we loved the story so much we would pretty much ignore the genre altogether and try to just continue reading the story and even if it were non fiction or fiction a majority of people would put their place in the main characters to want to get a glimpse of what it would be like