Whittier's final story was what the Nightmare Box showed its victims.
We know that the box convinced people that their lives were infinitely false, and all suffering was irrelevant. Perhaps somehow it implanted the same idea into them that Whittier used in his story: people only exist on Earth to "polish" themselves for the afterlife. But in real life, instead of being a joyous reason to die quickly, knowing this caused people to become numb and apathetic.The survivors will die shortly after the book ends.
They've been cooped up with Miss Sneezy, meaning that (if her story is true), they have all been infected with the Type 1 Keegan virus, which is fatal and uncurable. And if the final story is true, everyone or nearly everyone outside the building is already dead.The writers are all already infected by the Type 1 Keegan virus, and their brain tumours are causing their erratic behaviour.
Alternatively, the events of the book never actually happened, and the writers made it all up in the theatre with characters based on themselves.
In other words, the writers locked themselves in a theatre and wrote an in-universe version of Haunted.