those conspiracies don’t work most of the time “you can only keep a secret between two people, provided one of them is dead”.
the personal risk for anyone involved + the human psychological tendency to chat and to have a hard time holding on to immortal secrets mean it’s usually irrational for both organisations to do intentional cheating.
Regarding fences. In Israel, high quality fences were shown to be practically impenetrable. Except by trained and sophisticated soldiers of Hamas, different story!).
On the border with Egypt, there used to be thousands crossings. The fence took it down to ~10/year or less. In other 3 directions, too , Israel built fences that are pretty much blocking passage.
Is Poland’s “border wall” isn’t working, it’s because it’s not well built enough. And I understand the forest issues. Still decent walls can be built. And you install cameras and sensors and it’s pretty reliable.