Did you have a co-op session with a player whose game was set to Normal throughout your Hard Mode gameplay?

If you are sure you have not switched to Normal mode throughout the entire gameplay, then go to Play Campaign select your Hard difficulty save file and press the Advanced key indicated below the window with the save files. After that, choose the final quest and complete it one more time. The "Zombie Wannabe" and "Trick-or-Treater" outfits should appear in your character's stash. If that does not work, try completing a New Game Plus on Hard from beginning, please.