I got to the demon+slimes boss with just one character, I think I missed a trigger somewhere. I also didn't fight the succubus pair the first go so I think I somehow skipped that trigger too. It seems that holding the run button (Shift) or progressing through dialogue too fast seems to cause some events to behave strangely?
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.