I've seen accounts that the French plannned for the Ardennes as a "forced" avenue of approach and viewed it as a kill box? Is it true, and if so, where did they fail?
I've never heard of that at all.
The Ardennes were thinly defended (and the elite Belgian Chasseurs Ardennais, who the French thought would cover the area, went north into Central Belgium without telling the French what they were doing, leaving it even more thinly defended.)
The French were basically obsessed with getting as far into Belgium as possible and got obsessed with the idea the decisive battle would be in Central and Northern Belgium. They considered the Ardennes as poor tank territory (which they were, if defended. Even minimal air strikes killing ONE tank at the head of a column could have screwed up the whole German attack and even lost them the war.)
The 1940 Battle for France was a very close shave. The Germans very easily could have lost. Not only did the British and French have better and more tanks, and lots more artillery, they had more men, and arguably better trained men...and even the Luftwaffe wasn't nearly as superior to the French and British as they are made out to be.
And before the battle started the French arguably had better morale than the Germans, believe it or not.
And even after the attack started the Germans were lucky. There are several incidents you could point to where if they'd gone the other way the Allies would have won.
From November 1939 through May 1940 Hitler wanted to attack France; his Generals were sure Germany would lose, and kept getting him to postpone the attack every two weeks; the plan during most of that time was to attack through Belgium, exactly where the French expected it. The Ardennes attack wasn't adopted till the last minute, and only because the French captured the original German plans for the Belgium attack from a crashed airplane.