Inclusivity. Minor races were deliberately excluded from participating in law-making. On top of that, Humans had too much power.
So the key was to get rid of those corrupted, replace them, and include representatives of the minor races into the discussions of law-making.
It’s not perfect, but they try.
Absolutely none because none of the countries were on speaking terms afterward
Not exactly a law, but it was the absolute decision to create an oligarchy government for all areas. There are no countries, just continents. These continents are ruled by a single group of people called the Twelve Presidents. This was formed because the continental shift happened not only because of natural disasters but at the same time they were happening, multiple world wars were happening. This is something that would be similar to what happens during our timeline with governments not agreeing or being greedy. They would drop bombs that would wipe out an entire (though small) country. So the decision was made to prevent world wars, though it doesn’t help when my story is about a sorceress who destroys towns. 
There are small villages that do have their own leaders, but they’re typically hidden from the main society. The Twelve Presidents has a massive army stationed nearly everywhere that keeps things in check, similar to the Peacekeepers in the Hunger Games.