I agree that cohesion does similar things as courage would, but while cohesion is discipline, courage is willingness to put oneself in harms way. Thus, as it is in the game, cohesion can be brought close to if not all the way up to 100% through training alone. Courage, as it is stated here, could not be trained. It could only be earned in battle.
I, for one, wouldn't do it exactly like the OP. Instead, I would have courage be a number representing training, cohesion, and morale put together. It wouldn't change any results, but would rather show how many morale shocks your unit could sustain.
