Wizofoz, I think that JAR state that in the determination of Vmca and Vmcg you have to consider that this are Calibrated Airspeeds
Yes, Calibrated airspeed being Indicated airspeed corrected for Instrument error, NOT changes in Density altitude. You are, therefore, still incorrect in stating:-
But is also true that the rudder will need more air speed to control the aircraft's momentum.
It will need more TRUE airspeed, but the same Calibrated (and, therefore, for all practical purposes, Indicated Airspeed).
You don't seem to grasp the difference between TAS and CAS/IAS, the last being what the pilot needs to know in order to fly the aircraft.
Momentum is also an incorrect term here. The rudder is needed to counteract Thrust, which is a force, by producing an equal force in the opposite direction.