I take it that once a "Sniper Attack" card has been placed in a side's play area it can only be used once per "Action Phase".

I should have mentioned that the card was placed in the play area as a scenario condition of the "Liberation Scenario".

I was under the impression that it could only be used Once, and then discarded.

That is definitely my understanding of its normal use. My question is, does the instruction contained within the "Liberation" scenario - for the Germans to place the "Sniper Attack" card in their "play area" - change this? And if it does, how exactly is it changed? Can it be used once per game round? Or more often even?

I have a question about this card too: Do units still roll cover dice?

I do not think its changed. The sniper can be used once, and is discarded.

There is no cover against snipers.

The scenario setup only says that the German player has the card ready right from the beginning.