Really appreciate the work you've done on this and I understand about stair heights, but the file you created when imported into my version of SH3D it still does not create an L-shaped opening, just the usual rectangular opening. If the L-shaped opening works for you, then the file is fine and it must, therefore be either:
* SH3D program (v6.5 on Linux) has a bug
* I'm doing something wrong with the furniture import (I choose "Staircase")