I think once you deplete a card you lose any passive bonuses that are on it. To deplete you turn the card face down and facedown cards are considered out of play. From the RRG: "Deplete When a card is depleted, it is flipped facedown and cannot be used for the remainder of the mission."

I think the confusion is this: The terrain square itself isn't difficult / impassable; only the edges between squares are. So in that picture the storm trooper is being pushed through an edge that is difficult from the direction he is moving, but it would be impassable in the opposite direction. You can climb down into the sarlacc's mouth, but you can't come back up again.