I'm going to address the point about the world being governed by one government.
In a perfect world sure, it would be great; however, the world is not perfect so I say, a world ruled by one government would never work... You just have look at the avarage street to see why. There's far too much disagreement on the smallest levels which would throw off the larger levels from ever working. Neighbours disagree, there are disagreements in families, in towns, the whole country also disagrees which is why votes are needed to elect some one to a position. Even after that, there's always going to be people who are unhappy about how the vote turned out. Think about the recent presidential elections, sure you got a final decision, but I bet there are quite a lot of people who still aren't happy with the outcome. Trying to get global co-operation to work when there are all of these smaller problems stopping even the population in a country from getting along is just unthinkable.
Another problem that you're faced with is corruption. Having positions of power on a global level, there's no way it will be void of some sort of corruption.
Another thing that I'll point out is that there are already bodies that govern certain issues in the world such as The World Bank, The World Trade Organisation, a world health organisation, the international monetary fund, etc. At the moment this global economic governance isn't really working. One reason for this is that only a few countries have control over these global institutes which means that the 'developed world' has more and more control of the world's resources. I can't really go further into this myself because I've not looked that deep into it.
As much as I think that people won't be able to work together on a global level, I do believe that there's one thing that could make every one work together. A bigger threat. If you think about it, if a country is threatened by a bigger threat, every one basically works together in the country to hopefully over come said threat. So on a bigger scale, if the Earth had a bigger threat, then I reckon most of the arguements would be put aside for a bit whilst this threat was dealt with. In conclusion... Bring on the aliens @_@