Well, the USSC certainly used the first part that way, much to the surprise of the District Court I'm guessing.
"The staffer who wrote the syllabus for Miller seems to have found that the Second Amendment does not protect an individual right, but I see no reason why that person's opinion should bear any weight whatsoever"
Yeah, it never got that far. The Miller court didn't say one way or the other.
The appeals court that quashed the indictment against Miller and Layton did not feel any evidence needed to be examined to determine that Miller and Layton were innocent. If evidence had to be examined, that job would have been left to a trial court judge or jury.
The Supreme Court's decision didn't uphold a conviction; rather, it provided that the case must go forward to trial court where evidentiary questions could be resolved. As such, it would have been folly for the court to issue a decision with less than complete instructions. The entity responsible for deciding matters of fact would be a jury, and the only thing a jury could possibly do with information relevant to the defense is to acquit the defendant if they find that such information compels them to do so.
The clear implication, then, is that since the only matter the court mentioned as being in factual dispute was the military usefulness of a trenchgun, that would be the only matter a jury would have to decide in order to acquit.