Figured out what
Deaddarkness was talking about. You can avoid the black screen bug by getting Ending 3 (above 160) BUT staying under 300 hearts. After the ending CG, you'll get thrown back into the game keeping your current stats. Suspicion no longer matters and you can fuck her anywhere.
Ending 4 triggers at night automatically after reaching 300 hearts. The door should now appear after doing the maid events where there is one last annoying minigame for Ending 5.