meanwhile

Hello! I'm trying to model this shelf ceiling closet and running into a couple of snags. The shelf ceiling creates a strange hockey-stick-shaped hole through the other side of the wall it's against. How do I prevent this? It was framed unusually IRL – against the drywall, not into the studs behind it. How do I model this? (For what it's worth, the divider wall has three layers of drywall on the interior side.)