But only very specific parts. In fact, if aliens invade and you're in Paris or London, just stay away from the general vicinity of the Eiffel Tower or Big Ben and you'll be fine.

That could probably be said about anywhere, given time, money, and willingness to reduce population.

I could probably build an underground vertical farm in the Sahara and feed myself, if I pumped in saltwater to desalinate.

I mean, it absolutely could. California alone supplies the planet with a massive percentage of food. If the this image was accurate, we'd have a surplus of food. We might just have to tolerate losing a lot of stuff we can't (?) grow, like bananas. Or they'd be remarkably expensive because only Hawaii or Florida or somewhere that could grow them.

Power? Again, we have collosal amounts of energy or energy potential. From green energy to fossil fuels and natural gas. Without selling it to our allies, we'd have a surplus of energy.

This is literally why America is the superpower. But a lot of countries could likely do this. Russia could for sure too.

Economically, however, I can't even speculate.

Fun thought experiment, worthy of a Kutzgesagt video!

Edit: I thought you said "couldn't" but I already posted so 乁⁠༼⁠☯⁠‿⁠☯⁠✿⁠༽⁠ㄏ