How do you use .obj format to get parameterized windows for instance?

If you mean window information used to draw their arcs in the 2D plan, this additional information is stored in the properties file that describe each object in the catalog. See here for the list of available properties and download SH3F files for real world examples.

I saw a special code name for glass embedded in a .obj file. Is that the extent of special changes to .obj format?

I wouldn't call that an extent but a special usage in shape names that helped me to define mirrors from sweethome3d_window_mirror prefix and transparent panes from sweethome3d_window_pane prefix. Transparency can also be defined in MTL files, but MTL files weren't supported in the first version of Sweet Home 3D.
Recently, I used sweethome3d_window_pane again in Advanced rendering plug-in to ignore window panes during SunFlow rendering, because this rendering engine can't let light go through transparent objets, unless you use caustics which are very heavy to compute.

Is there more added to the .obj text file that helps you stretch a window, shelf, counter, etc?

Sweet Home 3D uses standard OBJ + MTL files (everything in OBJ format isn't probably supported, but what is supported is enough).

Is there a tutorial for window/door creation?

No but it's not complicated.

Is there a way to reuse such models with Blender? Other 3D editors?

You can export the 3D view to OBJ format to reuse your design in Blender.

What do you usually use to create furniture models?

I use Art of Illusion or Blender.

Are any of the usual ones in BREP form good for CAD?

I have no idea, I don't know what is the goal of your use of CAD.

What is used to make all those .3ds models listed at http://archive3d.net/ ?

Probably 3D Studio Max, but some other tools are able to export at 3DS format.