In order to make the beveled surfaces, I would use, in V9, Create-Surface-Ruled-Single.
Selecting the individual lines or arcs for EACH section of the wall.
You are correct stating you cannot maintain a constant 20 degree bevel with the current wireframe geometry.
This is the time you call the engineer and ask for a model for clarification or a detail view at the 45 degree point in the corners.
Don't be afraid to ask the engineer.
It's easier to deal with dumb questions than dumb mistakes.
ObrienDave. MasterCam since V6. Gcode since 1983.
The nose you punch today may belong to the butt you have to kiss tomorrow.