The arbitration panel ruled on this long ago. The US is in violation of NAFTA. The refusal to let these trucks in would be a "Takings". The damages have already been set at $2 billion per year and since this is already 2 years old it is $4 billion coming up on $6 billion.
Yes, the US can set standards and they have. It was taken care of in the DOT appropriations bill earlier this year. The inspection stations are all built and staffed.
So secret even they don't know what it is.