Also just to be sure, may on-table Artillery Support Units, Infantry Guns or Mortars be given fire orders by the command unit of any friendly unit insight of an enemy unit or must they be assigned to a particular unit and only that command unit can order them to fire?
I think you could start by applying the doctrine rules for this, i.e. rigid can only command their own supports, normal can use others with a penalty, flexible can use any. But it's really more complicated than that, and really depends on who, what, where and when.