I have noticed that I can use SceneViewer to view a vehicle geometry provided with HVE, but not an environment geometry. Why? How is a vehicle .h3d file different from an environment .h3d file?

The native HVE format .h3d file for a vehicle only contains descriptive general shape information in  Open Inventor (OIV) format.  Programs like SceneViewer can view any model in a "pure" OIV format.  The native HVE format .h3d file for an environment is also in Inventor format, but since it includes attributes assigned in the 3-D Editor, these files can only be read by HVE, not by other programs that cannot interpret this additional information.