Can there be a "Universal Militia" that isn't all volunteer, as opposed to compulsory?? Are they not one and the same?
You are missing the point. The "universal militia" is neither volunteer, nor compulsory. It's definitional. In other words, every citizen is
ipso facto a member of this militia, although no actual service is contemplated.
In 1791, this "universal militia" was real. It's no longer real. It hasn't existed in practice since the 1820's, by which time it had been found to be unworkable.
But it still exists as a notion. A very important notion, because laws and constitutional interpretations can be based on it. The concept of the "universal militia" gets around the conundrum of whether the 2nd Amendment describes an individual or a collective right. With the "universal militia," the individual is synonymous with, or is contained within, the collective, and vice versa. You cannot separate the two. The individual, then, can own and possess every weapon (i.e., military weapons) that can be possessed by the collective.
This line of thinking dovetails perfectly with the originalism of the conservative majority of the Supreme Court. They're looking for "text, history, and tradition" and this is certainly that.