The whole thing was Julia's plot
- Stauf represents Julia's monstrous side; when we see Julia die at the end, this is symbolism representing that her vicious side has won and she is no longer the person she originally was. She planned to sacrifice children in order to open a portal to another world/hell, but she invented the character of Stauf to be her "business persona" to reduce her feelings of guilt. (Initially Julia wanted to open this portal so that she could use its power to get her youth back, but eventually her cruel side takes over and wants open the portal just for the sake of making others miserable.) She kept her two personae purposefully separate and was already a bit crazy, hence her frivolous & manic nature when she's acting casual in her bedroom. She invited the other guests because their souls were needed to open the portal too. The details about Stauf's backstory are a mix/exaggeration of events from Julia's own past. This would explain why we see relatively little of Julia exploring the house (compared to the other guests).
Stauf still lives
- But not in the 'game world'... of course he might have you believe that you defeated him in some manner. Something stands out... the AI for Stauf's Reversi puzzle, which grows better as computers get more and more processing power... possibly just as he planned...