This is not about how far the case proceeded. There was never a trial because the trial court judge dismissed the case as a violation of the Second Amendment.
Now you are claiming that the Supreme Court erred by remanding only on the basis of the weapon's usefulness and in not specifying that proof of non-membership in a militia was also a requirement for prosecution.
Or do you recognize, as I do, that the Supreme Court was determined to rule against Miller, regardless of the meaning of the Second Amendment. Miller was a person and the people's right to keep and bear arms was not to be infringed. What is your explanation for the Supreme Court remanding on one issue but not on the other?
Was it so difficult for them to conceive of the possibility that the trial court judge would have to dismiss due to lack of evidence regarding the weapon?
Please describe, in light of the remand, how Miller's trial would have proceeded, who might have appealed, and what further rulings the Supreme Court might have made.
To me they're asking the defendant, "What does your possession of this weapon have to do with the preservation or efficiency of a well regulated militia?" (ie., First, what does this weapon have to do with a militia? Second, what does your possession of it have to do with a militia?)
Even if the defendants could have demonstrated that shotguns (<18") were militia type weapons, I think they still would have had to prove that their possession had something to do with a militia.
Ask yourself, why was the USSC so interested in the weapons relationship to a militia? If the second amendment protects an individual RKBA, who cares about a militia? Why ask if this weapon is a militia-type weapon?