There are scripts to make doors open and shit like that, but you can't make stuff walk on to an object and use it to get to other places. Units only walk on the actual terrain.
if you got the right skydome and made it so it has different levels and made a ontouch script (if this is possible in eaw) then u could do it fairly well actually making there be elevators it'd be cool
yes. that is possible, if you set only lV factories and barracks to be built and create new props. however, indoors locations are a limitation of the map editor.