Yes, Raoul and Christine were young once. Yes, most of the characters really existed. And yes, there was a Phantom-like man living in the labyrinth below the opera house. However, the main plot spanned many years; not a few months in the war-torn Paris of 1870-71. Because of his advanced age in 1919, Raoul's recollections of what really happened are sketchy. For that matter, some of his memories (e.g., the loud bangs of fireworks at the masquerade party, the armed guards in uniform, and the massive fire after the chandelier crash) seem to be him confusing events from the Siege of Paris with the story of himself, Christine, and the Phantom. The opera house was not even completed until the mid-1870s. Moreover, Raoul is overwhelmed with grief after Christine's recent death. His visible shakiness also suggests that he has a neurological condition, possibly one with cognitive decline. As a result, he cannot think clearly, much less recall everything that happened so long ago.
This would also explain why the film's timeline does not strictly adhere to that of the stage adaptation.
