The exchange above reminds me of Robin Hanson's criticism of the social science in Greg Egan's works.

Oh. So these communities, and Key's life, are extremely atypical of that world's humanity as a whole. That's something worth stating because the story doesn't even hint at it.

I disagree: it doesn't matter for the story whether the communities are typical or atypical for humanity as a whole, so mentioning it is unnecessary.

I'd be interested in hearing about how they handle telling young people about the wider world.

The relatively innocuous information about the wider world is there to read about on the earliest guidelists; less pleasant stuff gets added over time.

How do they handle people who want to go out and live there and who come back one day?

You can leave. That's fine. You can't come back without passing more tests. (They are very big on tests.)

How do they stop the governments of the nations where they actually live from enforcing laws locally?

They aren't politically components of other nations. The communities are all collectively one nation in lots of geographical parts.

Many people can. I just don't find it convincing that everyone could without there being quite a few unsatisfied people around.

They can leave. The communities are great for people whose priorities are being content and secure. Risk-takers and malcontents can strike off on their own.