Actually, the Miller Court indicated that "there is no judicial notice..." that a short barreled shotgun was protected by the 2A. This is because Miller, the plaintiff, was killed by some "friendly competition" and his lawyer saw no point in bringing any evidence to the table when: a) it didn't matter to his client; and b) he wasn't getting paid anything to do the work. The Solicitor General at the time put forth a specious argument about what the 2A meant, and the Court had nothing opposed to that view in front of it, so it adopted his view in their ruling. In point of fact, short-barreled shotties were commonly known during WW1 as "trench brooms," and could easily have been shown to have significant use and value to the militia.
As you noted, machine guns are highly valuable - though for the armed forces and NG, and thus also the militia. There won't be any lack of evidence today, that's for certain. My view of 922(o) is that it is, by itself, unconstitutional because Congress has passed a tax that the BATF cannot collect because of this subsection, i.e. the law contradicts itself. When combined with the fact that it limits the right to own full autos - an entire class of weapons - and that there are already about 250,000 full autos owned by civilians, there is no doubt that it is unconstitutional. Not to mention that the '34 NFA itself is a Constitutional abomination.
Looking forward to the day when us proles can legally by drop-in happy switches, and there are about 5 million of them out there - when that happens, there is NO WAY that any government is going to try to confiscate weapons.
I agree with everything you posted.