In addition, it would be very practical to have the list of walls and rooms in the same place as the furniture list. It would make selecting items much simpler. You'd be able to find everything at a glance and select different items easily.

That's something I have on my wish list too but in a little different format: Each list in it's own pane and free choice in show/hide and docking position for each pane. From what I understand from previous posts is that such a thing is not possible with the current User Interface. I would be happy with three panes (walls,rooms,furniture) on the left and the current top-left window on the right.
The alternative is what you suggest: integrate walls and rooms in the furniture list. A list that starts with three default nodes: Walls, Rooms, Furniture. Because the list will get longer the two panes on the left are better split into one left and one right allowing them the full height.

It's a difficult problem because other users will have other preferences. You can wait for someone to request to add polylines and dimensions to the list [:))]