Canada

Possibly (it's impossible to tell from outside the companies commissioning the goods where specific supply lines go), and yes, the law should be applied against companies using foreign prison labour unless it's been specifically worded to dodge that case. As for whether it has been so worded, well, I'm not a lawyer, but I can't see why it would be favourable for the legislature to do so, since they're not likely to get votes, Canadian jobs, or even much goodwill from the US out of allowing that kind of loophole.

That being said, someone will probably have to bring a court case to get the law enforced against firms using US prison labour specifically. I'd expect that to take anything from a few years to a couple of decades. Once it gets to that point, I expect that the courts will apply the law as written (which may or may not be the law as intended).