When something is possible, it is often not necessary. That it's possible that I may strike you in the nose if you're rude enough is generally enough to keep you from being rude enough to cause me to need to strike you. That revolution is possible is generally enough for governments not to trample too heavily on their population. In both cases, the actual act is not carried out, just the possibility of it is enough.
But I agree that the gulf between civilian and government might is fearfully large already, which makes moving to another country a much better option. Of course, your world won't have that option.
But you're back to the point of the OP - for all the people that want to exercise the option (e.g. some residents of China, North Korea or Iran) moving to another country isn't an option, so having separate countries isn't helping them anyway.
Also, since the EU has borders with free movement (and doesn't even check on internal borders), which meets the grandparent's goal of being able to easily move countries, you are wrong to say this inevitably means forming a single world government.
How old is the EU? Already, after only a few years, the erosion of sovereignty of the individual countries is well along. I think it proves the point more than refutes it.
About 50. Nations have chosen to delegate some parts of their running certainly - more so than the US states in some areas, less so in most others.
Any answer to the first point? This debate is immaterial since in countries that people actually want to leave: a. their own government won't let them leave and b. our governments won't let them come.
The question I suppose is, can we find a way to reduce our borders without creating a world government, since that is clearly a bad thing? I believe we can, will and should.
But I agree that the gulf between civilian and government might is fearfully large already, which makes moving to another country a much better option. Of course, your world won't have that option.