"Thus, modern nation-states completely abide by the Non-Agression-Principle. You only think they don't because you don't believe that they have ownership rights on their territory. Well, the onus is on you to prove that modern nation states do not rightfully claim their territory. IMO, the modern nation-state has a better claim to land than most individuals. Nobody in this world can claim that they have never violated NAP. Nobody in this world can definitively prove that their wealth isn't the result of stolen property, here or there. If nation-states cannot own their own territory, then your claim to your own territory is also suspect.

This is the deontologist's paradox. Ultimately, there's no higher authority in the universe to appeal to in order to convince all the other pesky humans that you've got a better claim than they do to such and such a parcel of land. It's always easier to go with established convention and that usually boils down to who can influence the most people to back his claim or credibly threaten to do so. You might distill that down to "might makes right," but that's just a crass simplification of how negotiation works. Anyhow, the moral fecundity of the system has nothing to do with how well it works. That is, if nation states happen to effectively own their territory, it does not follow that they own it because they have the preferable system and that the system is therefor objectively superior.

Government regulation and taxation is the result of a voluntary social contract that you have made with the government that, if you agree to live within this territory, you must follow these particular rules. Nullifying the contract is as simple as moving off of the territory, and formally renouncing your membership, as stipulated by the contract you implicitly agree to by remaining on the government's territory. All modern Western Democracies explicitly have ways for anybody to voluntarily cancel their membership.

This is simply "how republican democracy works," as written by a progressive liberal democratic youth. There's nothing about "this is how a rotary phone makes a call," that ~critiques~ a smartphone, laptop, or pager. They're all different means of accomplishing the same thing, in this case communication. Again, there's nothing here that suggest to the ancap reader that they're being critiqued unless one's ancap ideas are solely limited to some laundry list of moral qualms with liberal democracies.

Complaints that you don't have the power or wealth to buy your own sovereign territory falls on deaf ears, because your complaint highlights the most criticized aspect of anarcho-capitalism, that an ancap society would only guarantee freedom to the richest of our society."

Well that's actually a critique of ancap thought generally, but it's more than a little sophomoric. I mean, if the best you can come up with is "there's only going to be freedom for the rich," then you've clearly not even begun to consider how the present system actually works since that is one of the primary complains about liberal democracy, from liberals, democrats, and socialists generally. I certainly wouldn't say there's a satisfactory ancap rebuttal to that, certainly there isn't one that would satisfy a progressive who doesn't believe in the efficacy of market competition; however, there's absolutely no answer to this critique on the progressive side itself aside from changing the institutions of liberal democracy so in this vein we have what appears to be a contradictory argument.

tl;dr: One who claims that everything works well as-is cannot logically claim that "therefor everything else sucks by comparison," is valid. Anyone who criticizes modern nation states from a progressive vantage point cannot use the "it exists therefor it must be working," argument against other systems of governance provision because this is fundamentally at odds with how they approach government and society generally. It's disingenuous at best. That is to say, they too agree that there are better ways to remit the services of which government is now the sole provider.