I'm gonna try 2 LOD. It's not going to be that much more detailed so it wont help performance much, but It's just to try. As for the windows, if i make them transparent, all we'll see is an empty building through the windows so i'm not sure if it would improve the visual...
... not sure in this, but maybe an environment map "mimicking interior" ... have seen that on some games. But, im still think that glass reflection could be enough - but if its your model, you can put the 3DMax/blender file and maybe someone picks on the interiors too ...
Yes, pretty much something something ala EDF2025. Couldn't find a good picture, but this video shows the details on the buildings:
[Watch @ 3:18, 4:01, 4:29]
Basically what you described, a small simple interior textured behind the glass.