Hey, the first question, and it's a good one too
Quote:
13.52 MECHANICS: Overruns are conducted by moving no more than two units (EXCEPTION: Specialized units may overstack for overruns) through the same hexside into a hex containing enemy unit(s) at odds of 6:1 or greater
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So the answer is that Marines or Airborne don't count towards the 2 unit stack limit - this is part of what makes them so useful. But for airborne to be used in this way means they can't be airdropped onto the target hex directly. This is the main advantage of airborne.
If part of an attack doesn't come from across a river or line of fortification, the river defense or fortification advantage is eliminated for
all units making the attack. Airdropping an airborne unit directly onto the target as part of a combined arms assault can be a vital way of overcoming a strong line of defense.