I enjoy reading you. You're good at looking at things from different angles. But regarding states and citizens, I think most people assume the role of being a state bank when they begin holding FRN's. By not demanding that the currency be redeemed in public money, by default they are engaged in a private enterprise. It's voluntary, in that redemption is available, but they opt instead for using private credit.

So the servitude is voluntary and not forced, thus complying with the 13th Amendment. They accept being 14th Amendment citizens, and become subject to the jurisdiction of the United States.