Wrong again.
One minute you are claiming that declaring a law unConstitutional means that nobody can be prosecuted under it. Now you are suggesting that courts are going to ignore the Supreme Court when laws DO have unConstitutional reach.
When the District Court judge charges the jury, he will instruct them consistent with NFA 34 AND the Supreme Court Miller decision. He will explain that the NFA 34 cannot be used to infringe the right of the people to keep and bear arms that are useful to a militia. He will not instruct them that a person needs to be a militia-member to be protected.
It will be for the jury to decide whether the evidence presented at trial proves beyond a reasonable doubt that the short-barreled shotgun possessed by Miller and Layton is NOT useful to a militia. Only in that case should they return a guilty verdict. It is not going to be a case of "jury nullification". It is a case of "Supreme Court nullification".
If the NFA taxed "all arms not useful to a Militia" then, yes, a jury could hear testimony on short barreled shotguns and make their own decision. And their decision could be that the law, which is a valid law, simply didn't apply to the weapon in this case. Miller and Layton would be acquitted, yes, not because they had an individual right but because the law didn't apply to the weapon they carried.
But Congress specifically taxed shotguns with barrels less than 18".
Using the guidance provided by the U.S. Supreme Court, if it is determined that Miller's less than 18" shotgun IS suitable for Militia use, then the law itself is unconstitutional and the charges should be dismissed by the judge. If he doesn't and the jury acquits, then I agree with you that it would be "jury nullification with Supreme Court guidance and approval".