I'm not a constitutional scholar, so forgive me. What exactly do you think unambiguously demonstrates that the United States is not a democracy?
> ...and even by your definition, while an individual state might quality, the government of the USA does not: "supreme power" is wholly unavailable through the US government, at least in its legitimate purview (of course it exceeds this boundary frequently and flagrantly)
I feel like the definition addressed this. (In fact, given that this is Webster's, I'd expect that the definition was written to specifically include the United States government.) The definition says "the supreme power is vested in the people and exercised by them directly or indirectly". The fact that the Constitution reserves nonenumerated powers for "the states or the people" indicates a recognition that the power is vested in the people. The "legitimate purview", as you put it, consists of powers granted by the people who hold "supreme power".
I don't think there's a better source of information about whether the government of the united States of America is a democracy than the position papers written to advance the four corners of its founding instruments, and they deliver an unambiguous "no."
Let's say, for a a moment, that your definition, which seems to conflate these two distinguishable scenarios (one in which a government's power is strictly limited and enumerated, and another in which that power extends to the bounds of whatever a voting majority or their representatives might desire), is a reasonable one.
How, then, do you discuss this distinction?
...and even by your definition, while an individual state might quality, the government of the USA does not: "supreme power" is wholly unavailable through the US government, at least in its legitimate purview (of course it exceeds this boundary frequently and flagrantly). It has dominion only over the specific areas outlined in the constitution; those not mentioned are reserved for the states or the people.