Whether or not our brains are slowly improving isn't really too connected to the assumed genocidal aliens.

Anyway, on a couple of those points, we've actually discussed them before and again you are confusing commercially viable with doable. If I need a hammer, and there's one for sale for $5 and one for $50 which are effectively identical, I purchase the former, that doesn't mean the $50 one isn't 'worth it'. If I need to build a house with nails $50 is totally doable. Your assuming that some of the techs I've mentioned are somehow non-feasible or need improvements to work... that's simply not true, they're just $50 hammers. Same as I couldn't sell an old 286sx PC with DOS these days, that hardly means the thing isn't worth the money, it's simply not commercially viable because I can get way more bang with my buck. Something like a Seawater greenhouse is not currently commercially viable, that hardly means it doesn't work.

When we're discussing feeding people, you can pretty much effectively expand as long as one person can produce enough food for two or three people total, we just do more like 1:50 instead of 1:2 or 1:3 and that you have confusion about this issue doesn't mean we've got something to debate, it just means you don't understand the concept. Let's say a quarter of population just did agriculture inside greenhouses, they could easily produce sufficient biomass for themselves and the other 75% to eat and drive cars... that's not speculative and is in fact horribly pessimistic about people's productivity. That other 75% does the rest of the work currently performed by around 95-98% of the pop, also not an issue. In a greenhouse, even unheated, you get way more biomass per acre and you get way more calories per acre since we currently favor crops that yield maximum calories per manhour not maximum calories per land unit. I feel silly even discussing this, it's like explaining planets have elliptical orbits, also this is merely an example to show it's doable not the route that would be pursued. A CO2 powered gun is doable, for instance, and if we didn't have gunpowder for some reason we could use CO2, we wouldn't have to fall back on using swords and clubs. As long as gunpowder gets you more bang for your buck, pardon the pun, you use it, same as gas is better than ethanol, it doesn't mean we have to revert to using horses and buggies if we drain the oil reserves or that society will magically collapse as those reserves diminish. The smart man prepares in advance but there's a big difference between preparing in advance and predicting doom.

I also have no idea why when things like this come up, so many people hear number like 10 or 20 billion and start getting these sci-fi slumworlds or hive cities in mind... well of course I know where they get the idea from, sci-fi movies and pessimistic pundits. A pretty classy and spacey suburban home with 5 people living on a quarter acre of land has a population density of 13,000 people per square mile... there's 50 million square miles of land, you could house everyone in the peak of 1st world suburban spacious luxury and squeeze 20 billion people onto 3% of our land area and your typical classy 4 story apartment building with lawns and maybe roof gardens can squeeze in about 10x as many people without much complaints about elbow room. All that matters is if you can feed them without having to use so much of your manpower that's there's none leftover for specialization and non-working kids, elderly, disabled etc.

Just to bang home this point a little more because I feel like you're probably just not familiar enough with the concepts to override cultural indoctrination. Let's say I dump 100 people on a remote island ala Gilligan, limited freshwater, not too much land, just enough to feed them. There's constraint is that fishing and foraging, even if not capped entirely, will start producing limited returns in terms of calories per manhour, they will likely settle into a pop somewhere between 50-80% of their absolute maximum because that diminishing returns thing will let them get away with devoting a fair portion of their population to tasks besides acquiring food, where 5 people might produce X food but 10 only produces say 1.7 X food, etc. If they knock down the 1000 or so acres of jungle there and grow crops, they still have a freshwater issue, they need water for themselves and their plants, law of the minimum, on some different island that's really cold and rocky but gets lots of rain they have different minimums. All very straight forward and easy, now this is where things tend to stop these days. It's a pain to push the envelope when there's still plenty of available land that produces food cheaper and easier then your next step... if that's not available you take the next step as it is now commercially viable, that didn't mean previously it was impossible or even hard, as you seem to think. Sticking to our greenhouse example - which is but one of many avenues - these guys get their hands on a few thousand dollars worth of plastic sheeting and piping. They erect it, and now under those couple of dozen acres they actually produce sufficient freshwater from seawater to grow crops and more per acre, while the rain that would fall on those dumps onto uncovered crops or drinking cisterns. This may not have been commercially viable but so long as the amount of new food produced exceeds the amount of food people had to eat to erect it and pay for it and replace the plastic from time to time it is totally doable. It doesn't get done because someone will come by and point out that setting up a small cottage industry that will net them more food via sale and purchase per manhour than going into the greenhouse business. Same again, just because my PC is way better than the one I had in 1992 does not mean that if all I have is the old 1992 machine I am not going to use it as a word processor and spreadsheet instead of going back to pen and paper.

For the life of me, I have no idea why you seem to keep ignoring this, I'm quite sure we've had this conversation - even this specific example - on prior occasions.