Really? How would they prove the origin of each of the parts? The burden of proof is on them. This is a very difficult thing. This is what makes it unenforceable.
Again, there is a case on the books of them convicting some one. So to say it is unenforceable is empirically untrue.
How would they prove the origin of each of the parts
A bunch of them are very easy. The rest are not nearly as hard as you think. Receiver look at the import markings. Barrel is another fairly easy one as it will have some factory markings and characteristics of every other factory barrel as well as zero evidence that you have changed it. If you leave any of the factory furniture it is pretty easy to identify as well. Besides showing a jury a picture of a factory gun and making the obvious comparison, I suppose they could call expert witnesses as well as RRA or even Izmash employees to identify those products as made in good old Russia.
Again expert witnesses could establish where magazines and similar parts are from based on the characteristics of each countries (or even manufactures mags). There are only so many US made versions of such parts and so many foreign ones. Which is yours.
Certain OME FCG parts are readily identifiable. How many manufactures of US made FCG parts are there? Are they unique in any way? For example does it have a G2 stamp or USA or RSA stamp. If it doesn't have any of the markings or identifiers of the only US made parts then will a jury believe it is a US made part?
If I was investigating it I would pull your credit card records to see where you bought parts. From there tracking down where they came from wouldn't be that hard.
Further, as the authorities are making their case on all these things what is important in trials is what a jury will believe. If you have an expert witness saying part A or B is clearly a surplus part from Romania and you have no evidence that you bought it from Tapco (i.e. your's doesn't have tapco or USA or other markings that all the Tapco one's have) who is more credible and who is a jury going to believe.
Lastly, I'm sure they would love to be able to find internet postings of the person expressing their intent to violate the law.