Just a note... you don't really need to complete the game with a good ending, just any ending. (There are apparently 6.)
I stayed too long at the bottom of the steps in the alternate dimension and died. That was it for me. 100 GS. Since I'd saved using the picture upstairs, I didn't have much to redo. (Even if I had to restart, of course,I wouldn't have had much to redo.)
But no, you don't have to finish the story. I got my 1000 without even going through the final part.