Good explanation. To add a few points:
When visualizing how diplexors fit in, especially with regards to "you need a diplexor on both ends", just think in terms of a highway with entrance ramps and exit ramps. For example, the "primary" cable might be running from your LNB to your satellite receiver. This is the "highway". When you'd like to add your antenna signal to that same primary cable, you'll insert one diplexor near the LNB and antenna to combine them onto the single cable, and this is the "entrance ramp". Then somewhere down the line, probably right at your TV, you'll insert another diplexor, to separate them back out to the TV and the satellite receiver, and this is the "exit ramp". Corny, but it might help.
(In reality, a diplexor is just a special kind of combiner.)
DiSEqC is really the name of a technology, "Digital Satellite Equipment Control". DiSEqC-capable receivers are the controllers and they send digitally encoded commands to DiSEqC devices. The only two classes of devices I know of are switches, explained above, and positioner motors. A USALS motor is just a DiSEqC 2.0 (or is it 2.1?) motor with support for "GOTO X" command where X is an offset angle from true south.
Also, I believe multiswitches, at least DirecTV's, are all 22K switches. There was a nice one I was looking at last year that is a 2-to-8 switch, 2 LNBs to 8 receivers (or 8 DiSEqC switch inputs).
