Dan Kelly you are correct.
If a departing a/c had a x-wind from left to right , it would have a tendancy to want to weather cock into the wind thus yaw to the left. This is impart due to the airflow impinging on the tail. Thus right rudder would be required to keep the nose pointed straight down the runway. If an outboard up-wind engine (left) were to fail this would produce the most amount of unwanted
yaw with the least amount of rudder available due to a portion of the rudder deflection already used for x-wind directional control.
As for which is higher on the 747 Vmcg or Vmca. It is my understanding that Vmcg (on ground directional control) is higher. This is because the moment arm from the main landing gear to the rudder is shorter than from the CofG to the rudder. (Used in directional control in the air.) In addition when airborne you have the additional assistance of five degrees bank into the live engine.
Hope this helps,
Grind King