Are you sure you unlocked the Culture special ending with Culture at zero? I don't understand how that is possible. If you have a savegame before that happens I'd like to test it here
To answer you question: the endings are determined by the visual novel choices only. In the scheduler mode you also need to reach 100% relationship. Then, if you reach 100 in Culture (must be 100, so not 99) you also unlock the extra epilogue (both for Happy/Sad endings).
So what happened to you shouldn't be possible
edit: of course Culture is for Lawrence only, I meant the associated stat for each character