So even if you disagree with me about the nature of currency, our different views on currency do not have any effect on the condemnation of usurious loans denominated in those currencies
No. Usury and the creation of faux-wealth through usurious contracts is a pervasive problem in modern economies, and the nature of currency has not changed. However, even if we postulate that the nature of currency has changed, that does not alter the prohibition of usurious lending, properly understood.
It turns out that the kind of currency used is irrelevant to the issue of usury (Question 35), so various opinions about fiat currency, so-called “hard” currency, and other trading tokens or fungible commodities are entirely distinct from the subject of usury per se.
We exhort you not to listen to those who say that today the issue of usury is present in name only, since gain is almost always obtained from money given to another. How false is this opinion and how far removed from the truth! We can easily understand this if we consider that the nature of one contract differs from the nature of another. – Vix Pervenit
No. A sovereign guarantee is not the same thing as a personal guarantee. Two of the most important are that the sovereign is not a person but, qua sovereign, is an institution; and the sovereign has the power to issue currency. The sovereign may pay “interest” with tax receipts, but it is no part of the contract that he must do so; so even the notion that government debt intrinsically requires full recourse to taxpayers is wrong. The place to discuss this is in the linked post not here, because it is really off topic from the subject of usury. (Note: see also more recent discussion on related subjects here, here, and here).
It turns out that the kind of currency used is irrelevant to the issue of usury (Question 35), so various opinions about sovereign debt and fiat currency are entirely distinct from the subject of usury per se.
Sovereign debt was treated as something different from full recourse loans by the medievals, and the sovereign differs from individuals in several important ways
This doesn’t mean that the way our government is acting is wise, prudent, or even somewhere in the vicinity of sane. It just means that sovereign debt is not usury: it is a categorically different subject.
Many government practices may be not only imprudent but intrinsically immoral, without being usury. For example I’ve advanced a couple of arguments that property taxes are intrinsically unjust 
, and neither postulates that property taxes are usury strictly speaking, although the first draws on concepts related to usury.
This is exactly as we should expect it to be with a moral doctrine covering a particular species of sin. The moral prohibition of contraception is not in itself an all-encompassing theory of sexual immorality. Adultery and fornication are sexual sins distinct from contraception, and what is true in the sexual domain is also true in the domain of property: that theft and usury are distinct kinds of sins doesn’t make either particularly ambiguous. Neither the prohibition of theft nor the prohibition of usury constitute Theories of Everything about the moral use of property. Folks who attempt to turn the moral dmer for advancing their own broader economic theories do a disservice both to the doctrine and to their theories. That usury is a particular kind of sin and does not cover all sins in the domain of money and commerce was affirmed in Vix Pervenit (see Question 11).