Pick-a-boo_doo Yeah, the bookcase door did the same thing for me when I grabbed it. That is due to the light fixture you pointed out being placed with MOO on. It doesn't interfere with the function of the door so sims can come and go.
The build does use platforms in at least 2 places (might be one more, can't check atm). One is the main stage where the DJ booth is and another under the piano in the inaccessible room. Neither showed anything that would cause me to believe they had anything to do with the access to the 2nd floor problem.
Hope this helps.