Lower court cites of Miller are certainly a convoluted mess, but I find Miller to be pretty clear.
There certainly is a measure of error in the decision, however.
What went right?
Miller was granted "standing" as a person whose right to keep and bear arms was infringed by federal law. He was not required to show that he was in an "organized" militia. It was sufficient that he was a member of the "people".
The case was taken even though it was clearly a challenge on Second Amendment grounds.
The court didn't just shrug and say "sure there is a right to keep and bear arms, but that doesn't mean you don't have to buy a tax stamp".
The court didn't say, "but short-barreled shotguns are just so icky and might be used to kill innocent people, so they obviously are not protected".
Instead, the court improperly concerned itself with whether there was evidence that such a weapon could be useful to a militia. Had Miller been present to continue this process, his case would have been re-tried. If the prosecution, not the defense, failed to provide evidence that the shotgun was NOT useful, then the Supreme Court would be expected to either remand the case for a third trial or dismiss with prejudice.
Since trench-guns were used in WW I, it would not have been possible for the prosecution to provide evidence that such guns are not useful to a militia.
The only thing "convoluted" about Miller is the invention of the term "collective right" by anti-gunners and out-of-control lower courts in an attempt to suggest that Miller lacked the standing which the Supreme Court had already granted him.
Good explanation of Miller. The very limited holding of that case was that it is not within judicial notice that a shotgun is a weapon that could be useful to a militia. That seems to be a very easy standard for a defendant to meet.