I've killed all the monsters that Nicademus has asked me to (I know this because I can sleep and save) but no message pops up telling me I can leave. No stairway icon appears at the stairs to the painting. What do I do?

Check you journal. There should be a note.

Being able to sleep means nothing in this case. Have you opened all doors and checked all rooms?

Your best bet is to have the Auto-Pause at Enemy sighted switched on and center on party member. Then have someone strong wander about the place. Even if you have missed something earlier, the AI will not allow you to do so.
If the enemy is invisible ... you can only cuss at the developers for not having put in the True Sight spell.

Shot for the help. After my fifth go at the stage it just kinda worked out. I did it all with my tank and maybe it gave all the monsters time to surface. Anyway thanks for the help!