Stakes, once planted only apply to two hex sides, usually the front two. The other 4 hex sides remain open. When a line of point facing units have a line of stakes in front of them they are still open to attack from the side or rear.
Shooters can shoot as a priority target any enemy unit making contact with themselves and can change facing before shooting as long as they are not currently disrupted.
They can shoot at any enemy unit within range (usually 2 hexes), provided that they have a clear line of hexes to the target hex including enemy units making contact with other friendly units. Applying the 'half hex' rule is the best way of identifying legitimate targets - shooters must be able to see more than half the target hex from the hex from which they are shooting.
Any unit engaged in hand-to-hand combat either already occupying or fighting into broken ground has no impact bonus. Cavalry that occupy a broken ground hex fight as if disrupted.
The 'attacker' must fight every unit to which he has made hex-to-hex contact. When a single unit is attacking a line of units this means moving into contact with two enemy units.
I hope the above responses helps to clarify a few areas. happy HHM gaming!