let's say my onboard battery is stone dead (which I expect they are),
but I want to reset my camera back to its original factory defaults,
following the recommended procedure. if I follow the recommended
procedure while never UNplugging the camera from its "wall wart" power
supply, shouldn't the camera "return to" it's original default IP
address and hold that default IP address *ASSUMING* I've never unplugged
the wall power supply adapter from the camera?

After reset the camera software might perform a reboot and load back
the settings from the NV memory which is powered by the batteries. You
can see the status of these batteries by measuring their voltage, if
discharged I would try a good one.