Interesting reading. However one question and one observation. First the question:-
If the Vmcg and Vmca are indicated speeds why do they reduce with increase in altitude (especially density altitude)?
And the observation, (which is in addition to all that has been said about turning moments on the ground etc):
Vmca may also be less then Vmcg because the flexibility is available of putting on slight bank (5 degrees) to counter yaw due critical engine failure. This is not available on the ground hence lesser control deflection and hence lesser aerodynamic pressure or less IAS for the same effect is required .
However I just saw the graphs of an MD-11 in which the Vmcg is lower.