I already did this by reference to the "Italian" nation.

Yes, I understand how the argument applies to Italy - which is one of several European states that were created out of the synthesis of a number of independent kingdoms. But if you're offering the idea that nation-states are universally based on some imaginary unreality, I'd like to see you that would apply to a state that didn't have that kind of origin story. A state like Slovakia or Armenia, which wasn't a synthesis of numerous smaller groups but was deliberately drawn to provide self-determination and independence for a single group.

Every nation state is founded on these kinds of origin mythologies.

Every nation state? Really? On those specific types of origin mythologies. Again, it doesn't seem like that's how the people in Slovakia would describe the origin of their country. Or the folks in Denmark. Or the Thai. These countries weren't assembled out of constituent states after the Treaty of Westphalia. Some modern state were just the evolution of kingdoms that date back many centuries before 1648 (or whenever the Westphalian model came to their part of the world). Other modern states are of very recent vintage and were created out of struggles for independence or separation, motivated by the very real desire of a specific coherent group of people to have their own autonomy over their group.

The modern nation state has been a human catastrophe, and it remains one where ever the question of "who belongs" is the principle concern of the state.

That's such a weird position to take. It seems to me that reality is the exact opposite. The modern nation state is an amazing human invention, because it allowed for the first time the question of "who belongs" to be answered by something other than "just our tribe/clan/kin/religious brethren."

It's not like all the countries/kingdoms/fiefdoms (or whatever) before the modern nation state were models of inclusion, tolerance, and acceptance. England didn't need to be a modern nation state to kick us Jews out in 1290, or Spain in 1492. The question of "who belongs" has always been the principle concern of the "state" - even when the functional equivalent of the "state" didn't even remotely resemble the modern nation state. There was always some person or institution that was in charge, that got to decide "who belongs," and was deeply concerned with the question of "who belongs" - and who was more than willing to decide that execution/torture/banishment was a legitimate consequence of not being the right sort of "who belongs."

The existence of indigestible bits of people who aren't "who belongs" is not a product of the modern nation state - it's always existed. Whether the society deals with those indigestible bits of people in a humane way or an inhumane way isn't dependent on whether that society is organized in a modern nation state or a pre-modern kingdom (or tribe or clan), and I would wager that the modern nation states (despite their horrors) have a better record on the whole than the old way of dealing with them.