odd, since the succubus pair is an autorun event, unless you meant that you went to the boss without first rescuing sylphie after losing. but there should be an automatic loss event if you try to go to the boss room without her.
I never met her in my first run. I went in, retrieved the warehouse key, no succubus pair fight happened (IIRC I moved through the lower route between crates I think), I used the key, I navigated the warehouse (also pushed the boxes into place while there) and got to the boss, who I got to and then saw the bad end.