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".
You are trying to apply "logic" which differs from that of the Supreme Court in Miller. The Supreme Court guidance was to hold a trial and acquit if the weapon was useful to a militia. But to convict, if it was not. This is equivalent to saying, it is not a crime for Miller and Layton to keep an arm useful to a militia. But it is a crime for Miller and Layton to keep a firearm that IS NOT useful to a militia.
The Second Amendment does not protect arms, it protects people, and according to the Miller Court it only protects people in keeping SOME arms. But inanimate objects do not have rights.
The Miller Court was completely comfortable with simply constraining the existing law in particular cases but not in all cases. But it very explicitly protected the right of Miller and Layton to keep an arm useful to a militia. A militia-membership test was argued by the prosecution but such requirement was not granted by the ruling.
That is not how it is done. The usefulness of the shotgun to a militia is a factual matter, it is not a matter of law. The District Court judge is not going to decide whether the evidence does or does not establish the nature of a short-barreled shotgun. Dismissal is only going to happen if the judge recognized that the prosecution introduced NO EVIDENCE regarding the shotgun and such dismissal would be on the grounds of insufficient evidence. Otherwise, the jury is going to decide what the evidence means.
Also, the extent to which the NFA 34 is Constitutional was already spelled out by the Supreme Court. The law is constitutional when applied to arms NOT useful to a militia. It is unConstitutional when applied to arms which ARE useful to a militia. And no individual can be convicted under NFA 34 for possessing an arm which is useful to a militia.
The District Court judge has lost his authority to rule the NFA 34 Constitutional or not unless it is based on an issue which was not before the Miller Court. The militia-membership WAS before the court, thus that issue is no longer relevant to the case. If the prosecution dreamed up another basis for appealing prior to trial, the District Court judge MIGHT allow the appeal, but I don't think he is obligated legally to do so. The prosecution doesn't get an unlimited opportunity to delay the trial. The prosecution has to decide what they believe the law is and proceed to trial on that basis.
We do agree that the Miller Court nullified some application of NfA 34. We DO NOT agree that "jury nullification" is involved.
The District Court judge is obligated to guide the jury with regard to what the law is. The laws that apply are NFA 34 and the Second Amendment of the US Constitution. The jury will not be given the option of ignoring either one. They will have an obligation to apply both as dictated by the Supreme Court.
The jury's job will be to decide whether the evidence presented at trial establishes beyond a reasonable doubt that Miller or Layton committed a crime, given the District Court judge's guidance as to what constitutes the crime charged. The jury would be fully justified in acquitting if the short-barreled shotgun is found to be useful to a militia and the jury would be unjustified in finding otherwise.