Is it possible to take the Slingbox stream and input it into Windows Media Center, then use Media Center Extenders to view the streams? (ex:Digital TV box)
No. Monsoon Media (http://www.monsoonmultimedia.com) have boxes that can function as Networked Tuners for Windows Media Center.
I am looking to use 1 interface for my wife to learn and control all media products in the house for 2 to 4 locations.
Like a universal remote control? You might want to consider the Harmony line of remote controls provided by Logitech.