At the end of the happy ending, Simon say "They let me finish my book, and it has helped me a lot. I wrote a happy ending, just for myself. I feel better now." After his hospitalization, Simon continues writing te book. Simon's book finishes with a happy ending. That was the Coop ending.
In Coop, despite his misfortunes, everything turned out well. Of course, just the opposite is true. This overhappy ending only happened in Simon's book. He'll be in a wheelchair for the rest of his life, living in a hospital. It proved a mere consolation.
This explains characters such as the Apartments Predator, unknown callers, and suiciders, who have nothing to do with Simon's life.
Simon went insane while writing a gloomy novel, spoiled rotten his whole life. The lesson is not to write dark things when you are clinically depressed.
To recap, the first path has David driving a car and, following a nightmare sequence, crashes his car in the woods. Alternatively, he crashes into Simon which knocks David out of his hallucinatory state, and books it when Simon starts yelling at him.
