Agreed. If they had fought off the Normandy landings, there may have been a small chance that Germany could have stalemated the Soviets and got an armistice. But once the western Allies had a solid foothold in Europe, the Germans had no chance at all. They would lose the war regardless of whether they won or lost in the Ardennes.It wouldn't have changed the outcome. Even if they managed to win the battle, they had depleted their armor, air support, everything in getting there. The hope of the win (that the US/Britian would negotiate a peace treaty and leave them alone) was never happening whether they were victorious in that battle or not. Allied forces would have suffered a LOT more casualties, but I don't think would have negotiated at that point, just brought in more reserves that Germany didn't have for the next fight.
Anyway, there was no chance that the Wehrmacht could have pulled off Wacht am Rhein. It was too bloody ambitious. They might have been able to achieve Fall Martin, but then again, that would only offer a minor improvement to their strategic position. Hitler knew that he needed a major improvement to have any chance of saving his regime.