Currency has been an objective UNIT most of human history.
I'm sorry, but, once again, this is simply wrong.
I would get out my hundred-year-old Black's Law Dictionary and quote from the definition of "currency" and the half-dozen cases that explain in detail why currency, not only the concept but the actual etymology of the word itself, is subjectively determined by market acceptance, but I've posted it here before and I'm not sure that would convince you, regardless. Just search my posts in the legal section if you're interested. Or we could go through the hundreds of years of human history during which currency was not constant, and catalogue the millions of people who lived and died their entire lives using currency that was not an objective unit of value. Or we can point to any of the dozens of fiat currencies in existence today whose values change continuously, yet which billions of people use as currencies regardless. But we've already done that, I think, and it didn't convince you either. So, frankly I can't say there's much left to point out. Your ridiculous view of currency as a defined "unit" requires someone to back that definition against the collective force of billions of market participants, which is objectively impossible. Stick with Federal Reserve Notes if you think otherwise. That sounds like the closest thing to what you seem to want anyways. Good luck.
If you have a different definition of currency I would love to see it.
links?
COIN, n. Pieces of gold, silver, or other metal, fashioned into a prescribed shape, weight, and degree of fineness, and stamped, by authority of government, with certain marks and devices, and put into circulation as money at a fixed value, Com. v. Gallagher, 16 Gray, Mass., 240; Latham v. U.S., 1 Ct.Cl. 150; Borie v. Trott, 5 Phila., Pa., 403
Furthermore, coin, money, and currency at least (of the terms you listed) each have distinct definitions that pre-date any US government: (emphasis mine)
Strictly speaking, coin differs from money, as the species differs from the genus. Money is any matter, whether metal, paper, beads, shells, etc., which has currency as a medium in commerce. Coin is a particular species, always made of metal, and struck according to a certain process called "coinage." Wharton.
Coin is just one type of money. Money is that which has currency. Currency is
current value. Pop Tarts constitute currency in some places. There is absolutely no reason not to refer to Bitcoins as
currency.
You keep wanting to claim that
currency is a
unit. That's etymologically incorrect.
Commodity is the unit, because a commodity is a bunch of items that
are all functionally equivalent. They can be used as units because they have
common value. Currency is that which
circulates as a medium of exchange, like a
current.
idiot. 
