For me, it's pretty clear from my original post that the government is far from a morally legitimate issuer of currency.  "Legal" legitimacy is really a circular argument, so I won't write the War and Peace on that.

The behavior of money has been extremely diverse in the history of civilization.  When China finally disavowed state issued money, for centuries, money in circulation consisted solely of unmarked silver ingots that could be weighed and cut on the street.  During 15th century Europe "money" besides physical gold and silver coins consisted of accounting entries of the powerful private banks, an early form of credit money, if you will.

If the history of money shows anything, it is that any "theory of money" is highly suspect, and is usually the intellectual propaganda of the powers that be (who enjoy the privilege of issuing money) through their mouthpiece, the economics profession.  The latter has multiple incentives to work with the elites rather than point out their use of money to receive unearned wealth and power.  The proof in the pudding is that, through the centuries, the elites themselves have changed the "nature of money" several times.