Surviving challenges in a tunnel due to meteorite impacts and planning for self-sustainability

Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: Surviving Seveneves, published by Yair Halberstadt on June 19, 2024 on LessWrong.
Contains spoilers for the first couple of chapters of Seveneves
Highly speculative on my part, I know very little about most of these topics
In Seveneves Neal Stephenson does the classic sci-fi trick of assuming that exactly one thing in the universe is different, and seeing where that takes us. In his case that one thing is the moon has somehow exploded.
And where that takes us is the complete destruction of the earth. As the initially huge chunks of moon rock hit into each other they break into smaller and smaller pieces, and take up more and more space. Eventually this process increases exponentially, the loosely held collection of rocks that was the moon disperses into a planetary ring, and earth is bombarded by lunar leavings for 5000 years:
There will be so many [meteors] that they will merge into a dome of fire that will set aflame anything that can see it. The entire surface of the Earth is going to be sterilized. Glaciers will boil. The only way to survive is to get away from the atmosphere. Go underground, or go into space.
They have only two years to prepare. Which option should they take? The choice seems obvious!
But they respond with the absolutely batshit insane solution. They go into space. And not to mars, or some other friendly location. Low Earth Orbit..
This is a terrible choice for all sorts of reasons:
1. They are even more at risk of meteor collision there, since all meteors that hit earth pass through LEO, but at least the atmosphere protects earth from the small ones.
2. There's simply no way to get people up there at scale. No matter how you slice it, at most an insignificant fraction of people can get to LEO. We simply don't have the capacity to send rockets at scale, and two years is not enough time to develop and scale the technology enough to make a dent in the 7 billion people on earth.
3. To prepare as well as possible in two years, the earth economy will have to keep running and sending stuff up to space. But if people know they are going to die, and don't have any real chance of being one of the lucky survivors, why would they bother? I would expect the economy to collapse fairly rapidly, followed by looting, and the collapse of government structures.
4. There's a thousand things that can kill you in space, and just staying alive requires lots of advanced technology. If society isn't able to keep a highly technologically advanced society going in space, everyone will die.
5. Keeping a technologically advanced society going with a small number of people is essentially impossible.
6. Earth technology and processes often don't work in space since they rely on gravity. New technological processes will need to be developed just for space, but with only a tiny number of people able to work on them and extremely limited resources.
7. There are no new resources in LEO. There'll have to be 100% perfect recycling of the resources sent up from earth. But propellant has to be expelled every time they manoeuvre to avoid meteors, so this is impossible.
Stephenson works with these constraints, and comes up with what are IMO wildly optimistic assumptions about how society could function. Whatever. But the obvious solution, is to just go underground, which doesn't suffer from any of these problems:
1. The ground + atmosphere protects them from all but the largest of meteors.
2. Digging is well understood technology, and we can do it at scale. There's no reason why we wouldn't be able to create enough space underground for hundreds of millions, or even billions, of people in two years if everyone's lives depended on it.
3. Since people know they can survive, there are strong incentives to keep working, especially if money will be needed to buy one of the spaces in the un...