Mackie extender without main unit?

You can do it in Cubase I know.  I had a control and two extenders (have since changed to Avid Artist), and it came down to the order that each unit was defined in the devices screens.  If a unit was missing (not turned on, etc), then Cubase would launch without it.  The extender would still control whatever faders (I think it started with the first 8 tracks, but can't swear to it; it could have been the last 8).
 
Unfortunately, I can't speak for Sonar and I don't have the old units hooked up anyway so I can't test it.

MC/Extender/C4 are completely separate devices in hardware. I mean there is no communication between them, not even throw software.
 
Mackie plug-in in Sonar is "the one" DLL for all devices. It has DLL global area for common information (mode, mod keys, etc.). So without MCU it is not possible to switch modes and use mod keys (till you have some other device which can imitate MCU is that respect). But I think it should work the same way as in Cubase without any tricks.
 
Since the functionality is limited in that case, you get the same result with simple configuration inside AZ Controller. The difference is that you can "combine" any devices, so you can for example use any controller with buttons/pads to switch controlled strips, change encoders resolution or put XT into "ACT mode".