Third: Tatsuya is adopted.
Fifth: Don't think this is so much a puzzler as an implication that the author is raped/killed at the end.
No, something is weird here. He skips breakfast, forgets to lock the door, gets yelled at by his girl-hate-inducing boss, gets home, unlocks the door to his thrashed apartment, skips breakfast again, and goes to his bedroom. Not sure what's going on, but him skipping breakfast twice and never saying he goes to work makes me wonder if he's actually the one thrashing the apartment and stalking himself, some form of split personality thing.
Eighth: Old man traps him in the box.
The old man never tells him how much prize money he gets. Presumably, it's something less than $50 and the old man has just ripped him off for the difference of the two amounts. The prize money is right in front of him, in the old man's hand.
Tenth: How do you get a straight on picture of a mirror of an empty bathroom, without the photographer in it?
Eleventh: No idea.