My mistake about the war in Ukraine

The soaring living standards over the last few centuries is the result of rapid technological advance, which I don't dispute. What I mean by "overpopulation" is that we would be even better off, have a higher per capita income, if there was less people at the given tech level.

We wouldn't have the tech level that we have now without billions of people, but I'll save that argument for later. 

Here is a more relevant critique of your position: Notice however that the issue here is per capita income. The relevant metric for wartime capability is a nation's total productive capacity. Total productive capacity of a nation does increase with population and we can prove this with a simple thought experiment.

New babies are not born with a ticket that entitles them to a percent share of world GDP or land or iron ore. In practice they get a lot of support from their parents if they are to survive, but that's not really relevant for the purposes of this thought experiment because we can imagine them now at 18 years old and kicked out of the house with no money. Since on average regular people, even people who start out without assets, still find ways to sell their labor on the market for more than it takes for them to survive, we can assume that, on the margin, more workers contribute to China's economic production. Even if your hypothesis is correct and their existence lowers per capita wealth statistics, they're not "taking" that land from other people just by being alive - they have to work and trade for it like everybody else. That's capitalism.

Imagine what would happen if we dialed world population to extremes: 

Ok.

At one end, you would be the only person in the world. Theoretically, all the world's resources belong to you, making you very rich. But in practice, you will live an impoverished and short life, because you can't run a modern economy all by yourself to make use of those resources.

No, you're overstating the case. Most of those resources wouldn't belong to you; at least, they'd remain unavailable to you. This is an important distinction. You wouldn't even "own" any of those resources because you wouldn't have the manpower to actually dig them out of the ground or capture them. You would not be able to get any of the sunlight by building solar panels, or extract the coil and oil from the ground. The only resource you'd instantly "own" in a trivial sense would be the stuff that you could walk to and find some way to pick up, like the land in a certain radius around wherever you were.

When you imagine this scenario, I want to make sure you're not imagining a scenario where everybody in the world disappears, as that's not an analogous example. The actual world is filled with secondary goods that people built, not just premined coal and oil. The default one person world in your mind's eye should be Eliezer Yudkowsky transported to 10,000 B.C.E.

At the other end, the world is populated by 200 billion people and everyone is on the brink of starvation, surviving in urban slums on a diet of insects and algae.

This seems like an arbitrary number and I don't think you actually have any mechanical reason to believe this is the case. I would like to know what you believe are the bottlenecks on, for example, food production and city planning. Why wouldn't we be able to just scale up the solutions we already have? 

Somewhere in the middle, there is an optimum level of population that maximizes wealth per capita. This dynamic is a version of the Laffer curve, which tends to arise whenever two opposing factors are at play: For government tax revenue, it is the diminishing incentive to create wealth vs. the expanding fraction of wealth captured by the state. For aerodynamic resistance in rocketry, it is the decreasing air density vs. the increasing speed of the vehicle. For world's per capita income, it is the shrinking share of resources per person vs. the soaring economy of scale in production.

Again, this model doesn't account for the fact that new people can do things like build solar panels, farms, and oil drilling plants that actually "increase" the amount of "basic" resources that the rest of the world has access to. Innovations that increase the yield or make use of these resources better can also functionally be considered the same thing. 

In a further step, I believe there is overpopulation (i.e. oversupply of labor) at all levels, from unskilled labor all the way to the Einsteins of the world. I think the world's best scientists/engineers are mostly positional goods, similar to the world's best athletes - in a counterfactual world where they never existed, things would be chugging along at the same pace. This is why I'm not worried at all about the staggering number of university graduates and engineers China is producing each year - all this does is intensify internal competition.

You just state this is "what you believe" and don't explain why you believe it.  There are plenty of obscenely rich people in Silicon Valley today who would not win a positional game on business leadership in a million years but got lucky picking a correct idea. Larry and Sergei did not become billionaires by beating other entrepreneurs at a positional game - they became billionaires by creating a trillion-dollar search engine. And if your society mints 50% less entrepreneurs or engineers, you have 50% less opportunities for one of them to spend the trivial amount of resources coding and deploying a test version of Google. The faster the microchip/the microwave/the radio/etc. is developed the faster spinoff technologies can be developed and the more information the rest of the nation has on the search space for new tech. This is how "more people" turns into "proportionally more growth"; not only from the engineering and efficiency gains from regular engineers at Uber/AirBnB/Boeing but also the increased amount of rando startup founders who try their hand at making meme companies.

Perhaps you think scientific understanding is bottlenecked somehow by acceptance from a top cadre of "thought leaders", and I don't think I'd disagree in principle, but you also have to admit that it doesn't seem like the top minds of Ancient Greece don't seem like they would be able to handle building the atom bomb.

For this reason, I see the post-WW2 globalization of economy  as largely a redistribution scheme. I still think it was worthwhile, but only on moral grounds rather than economic self-interest of the powers that promoted globalization. Urbanization and rising living standards are among the most effective methods of hampering population growth, alleviating the problem at the source. Globalization has also been a promoter of peace, thus avoiding burning resources for nothing in wars.

I'm at a loss as to how this is supposed to follow. Do you think globalization causes poorer countries to increase in population and thus use up more global resources? But then this goes back to the fallacy that new people come with a share of world per capita income. The Chinese aren't using up land Americans owned before they started trading with them.

It's also just patently false in the short term - when the "post-WW2 globalization of the economy" ceased for Russia it made them poorer.

I'm thinking #1 cannot possibly be your actual view. Are you trolling? What do you think the "schelling" army+support staff size that can defeat any larger army is?