1: Shadows in 3D window:
Save the image twice - as .2dl - with the building image separate from its shadows. This option is in the save dialog.
Merge the files back into your plan or place them into an independant detail window.
Use the separate shadow image to establish fill boundaries.
Paste the shadow fills over the building image.
2: Shadow intensity: The OpenGL view, at least, responds to sun and ambient light levels, so you can tint the shadow by using sun and ambient color and also make the shadow weaker. Have fun.
Dwight Atkinson