Hello Simon.
Well these are my personal answers for your questions.
1) to have open doors you simply need to use the open door object, although it will remain fixed open since SH3d cannot manage the angle of the door leaf
2) if you plan to use different colors or textures for a same wall on each floor, then definitely you need to redraw the walls
3) if the shape of the area under the stairs is simple, for example a triangle, you can draw the wall from the floor and then insert transparent triangle object which you previously need to import marking it as a door/window, so it can "cut" the wall where it's inserted
I hope this helps.
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If a long walk every day were healthy, mailmen would be immortal.
