(1) Currency is a zero-coupon perpetuity, so it is constitutional. The Fed regulates it, so the Fed is constitutional. What may not be constitutional it making currency legal tender. Imagine what the economy would be like if only metallic money were legal tender. No currency, no checks, no credit cards.
(2) Kieffer confuses the monetary base (currency) with money (MZM). MZM growth has been average since 2008. The Fed prevented a collapse of the money supply and the financial system by injecting currency to offset the huge drop in the money multiplier since late 2008. Inflation has been lower since 2008 than over the previous 10 years. The exchange rate now is about the same as at the end of 2008. What's the problem?