Making lots of treaties is better. You have flexibility, some measure of competition to keep the economic juices flowing, and, most of all, each nation gets to decide for itself what best serves its own interests. One monolithic law not only consolidates power in a body that may not have the first clue what it's doing, but also tries to make one size fit all.
No, treaties can be broken. The United States broke every one with the Native Americans they ever signed. Each nation will still have it's parliament to handle local issues, but will agree to be bound by what the central body, the one world government, decides on issue like international trade and perhaps defense.
Again, this is only unsubstantiated conjecture and is belied by the very real problems of culture, tribalism and geography
The only real problem you listed is geography, and it's becoming less of a problem each day. Culture and "tribalism" are completely socially manufactured and irrelevant.
Do you really think "culture" tribalism and geography are serious barriers to anything? You must not be familiar with this:
Kathmandu, Himalayan mountains, the Roof of the World
People in Kathmandu are powerfully aware of living in a radically new era. Whereas the grandparents (and even parents) in this wedding story grew up at a time when communications with the world outside the Kathmandu valley required weeks of grueling overland travel, the bride and groom grew up watching global media events like the Gulf War and the World Cup "live" on television.
People born since 1951 have witnessed the world arriving along the first motorable roads into the valley; through telephones and now satellite telecommunications; through electronic entertainment media (cinema, television, video, satellite TV); via air transportation, mass tourism, and a surge of global commodity imports; and through the logics of a new bureaucratic state apparatus, party politics, and large-scale foreign development aid.
http://www.pupress.princeton.edu/chapters/s7365.html
Everyone in the world recognizes that governments vary wildly in quality and everyone wants the best government they can get.
That's unless you name one group, anywhere in the world, and provide the link please, that has stated:
"We want bad government! Give it to us now! Give us more corruption or we're going to start rioting!!!"
The Soviet Union could not get everyone in their empire to act with one mind. The Chinese couldn't do it either.
That's because the Soviet Union and China used force to get their way.
Do you not understand the difference between choices made at gunpoint and choices made by debate and voting? There is an incredibly large difference between those.
There is no way a one world government could become a tyranny. There are so many mechanisms available to prevent something like that from happening, it is not at all realistic as a probability.