I'd point out that pretty much all social interaction is based on more or less the same concept as nationhood. Also, while I know a lot of people view a united Earth as a future outcome, especially in an inter planetary context, I find it very unrealistic. For one thing I doubt we'll ever colonize that many planets when its probably a hell of a lot easier to build them instead - I don't think people really appreciate how much energy is involved in even slow-boat interstellar travel compared to, say, disassembling a few asteroids to produce a few billion of these). And for another in terms of colonies, especially absent native exploitation and colonization by dissident factions, a mega-country is much more like to form around the US and its couple of hundred orbital/asteroid colonies then the US with Mexico, I'd wager.

If families acted with personal self-interest in mind there wouldn't be many families to begin with.

Just going to point out that default human culture is tribe/village, not nuclear families, and democracy is generally only a good governing method, like most governing methods, where the governed don't all know each other very well.

You'll leaving out that these tribes routinely interacted with each other and the nature of the industry did not require unified rule, just a loose agreement not to hunt each other's lands and occasional trade or right of passage. Early nations typically spawned out of water rights, hydraulic despotism, places where people were packed together and where someone would inevitably have control over the river feeding the flood plain, democracy rarely featured in those arrangements.

I'm not sure why, space isn't really set up for border disputes, moving the boundary between two stars a couple billion miles isn't forfeiting valuable real estate, and there is absolutely no item worth trading between stars systems except information. Continental politics are a bad basis for interstellar ones, tiny islands on the vast pacific ocean better fit the bill, and are very different.

Well the progression is flawed, IMHO, especially because it does delve into sci-fi land. Humans aren't going to rally around a unified government because of aliens unless there's a real perceived threat, and to be blunt, if there was we'd all shortly thereafter be dead. Any species capable of crossing the interstellar void could crush us like a bug. They don't need to invade us or bombs us, all they need to do is not bother decelerating any ship they send and just let it hit the planet. If some aircraft-carrier-sized ship hits old Earth at half the speed of light it would be worse then a few million nuclear bombs going off. The flipside of that is that if you have power sources capable of doing that planets, and even stars, aren't actually valuable real estate anymore, because they don't really have anything you need that isn't more readily available elsewhere.

You get to that scale you may as well be modeling human civilization by looking at ant hives and trying to figure out why they don't seem to develop a postal service and souvenir shops. There's pretty obviously no inter-galactic governments, I'd be very damn surprised if there are even inter-stellar ones, doubly so if there wasn't FTL options.