That's a little easier said then done Planets are not very defensible positions.

It could form, or a near-total one could form, I just think it would tend to dissolve without constant external pressure largely because it really does not have many benefits that actually hold up under scrutiny.

I think that it would be very hard to stop anyone from doing their own thing and that it would very hard to offer them an incentive to do otherwise anyway. And again, I think 'interplanetary' is as big a misnomer as to refer to modern civilization as 'inter-cave'

I think most of future humanity, barring a major change to humanity, and assuming we get functional fusion, is going to live in little habits about the size of county or medium-small state, essentially a 'micro' or 'milli' world, that was built on commission from some big company, towed to the place they wanted, and occasionally needs to buy some hydrogen and metals from outside to keep their tanks where they wanted, or will just have very big tanks. After all you could provide day-time light for one the size of the UK with about 50 or so Terawatts, which is like 10 kilograms of fusion fuel a minute or about 5000 tons a year. So wrapping the enitre island round like a cylinder, but one with a quarter million square kilometers of area, even if the hull just had a meter thick of the gas at STP acting as storage tank and radiation shielding, we'd be talking about 25,000,000,000 tons of hydrogen, enough to run the place for 50 million years, and that if you only went a meter thick and didn't pressurize it. The size of the colony doesn't really matter either, since power for an Earth-facsimile is linear to land area and fuel storage is linear to area if its just a layer of hull, it's all how thick and how pressurized? A meter at STP grants you 50 million years between refueling, double the pressure, double the time, double the thickness, double the time, and there's no particular reason one couldn't be built 10 times more pressurized and ten times wider, and thus have a life of 5 billion years, longer than a sun, between refuelings. You could probably screw around with your external symmetry so you didn't uniformly radiate heat in all directions and have yourself a handy if slow star-drive shoving you between star systems on - in this case - several terawatts of IR photons and to top it off so long as you're going slower then the break-even point of ramming into loose hydrogen gas, in terms of power generated via fusion versus kinetic energy absorbed running into it you could just hang 'bags' on the side to scoop up hydrogen as you went. You could comfortably drift through the galaxy at 1% of the speed of light just refueling as you went, no ramscoop needed, maybe not even needing to pause to get mineral ore since one can probably use fusion to make it anyway.

So we don't really have a hold on people, if the amish want to buy a county sized hab to circle the solar system a few hundred AU out, then yeah, they'd need to keep ties to someone for maintenance purposes presumably, but that's not too likely to be an issue for anyone else. There's also no reason a few thousand people couldn't get together to form Marx-hab or Ayn-Rand-ville or freaky-cult-place, then your issue is whether or not you can enforce certain basic freedoms if they just slam the door shut, tell you to go to hell, and the governing body of the thing remind everyone they can vaporize the entire micro-world if the heathens try to invade and will rather than blah, blah, blah and again a habitat is basically a spaceship by defintion, so the crazy cult can always slow-boat to the other side of the galaxy.

I ran through the numbers not to bore anyone, but rather because sci-fi leaves them out so often people don't get the absurdity of what's being suggested. We don't what future tech is going to look like. We know fusion is possible because stars do it and it can be done in a lab, and because its just 'more power' so its not as hard to guess the effects as it is for something like radio, TV, internet, etc. We might not even need that route of colonization. We might digitize ourselves to a place where physics doesn't matter and we just happily simulate an 'Earth' several trillion miles in radius rather than several thousand while all sitting on an actual Earth where robots, remote controlled or AI cheerfully do the maintenance work. I always keep my speculations to 'basically current human' because the alternative is raw crystal ball guesses. In thats sort of context we don't need much more new tech to reach a point where people can just chip in to buy a habitat for a few thousand people the size of a nice town or one for a few million the size of a small state, to something really compact where the food is strictly hydroponic. I think I worked out once that if you went for photosynthesis-only spectra at optimum lighting you only needed 25 W/m2 for growth and only about a hundred square meters per person. Big-ass luxury worlds emulating Earth complete with dirt and forest and lakes might be expensive but the aforementioned wouldn't be.

So it comes back to, for unified governments, 1) What purpose would they serve? 2) How much leverage does that service yield?, 3) What sort of options for involuntary inclusion do you have? Predicting the future is always an exercise in futility but its really hard for me to imagine the Town of Charon 12, a nice little habitat of 15,000 a mile long floating around Pluto, having any interest in sending taxes or submitting to laws from elsewhere. Now, fuel, raw materials, luxury items, information, yeah, but those first two can be gotten anywhere and the other two are kind of optional. Charon-12-ites don't really need a laser feed from Earth sending 10-hour old news and Iron Man 98 to them. They might want to have an arrangement for security, some private insurance firm if the hab gets a huge hole punched in it, they might rely on some fiat currency and banking backed by some economic titans, but they really don't need much and they're likely to have a lot of alternate suppliers.