You seem to be using tap rules to justify undersizing the egc.
Without getting into the issue of 'are taps allowed in this circuit', _if_ a tap were to be allowed I could see your point that the egc could be reduced in size to match the size of the taps.
For example, if one had a 50A cooking appliance circuit feeding a 20A cooktop and a 30A oven, I could see your justification used to run a 12ga EGC to the cooktop along with the 12ga supply conductors.
But in this case, we don't have a tap. We have the full 10ga 30A supply conductors feeding a receptacle. The egc must be sized for the 30A circuit, as per table 250.122.
-Jon
