A lot of those questions are simply not answerable with our current knowledge. All I really have to say on the issue is that it has been demonstrated that we need centralized governments, and if the day ever comes when we don't, it will likely be a day when there either couldn't be an incentive or the fail-safes are in place for countering extreme disturbances like you mentioned. That day would probably come at a certain tech level and cultural integration

It's like how there was once two world wars started in Europe, but today there couldn't be. Certain things have happened between now and then that have changed the European dynamic. At some later date, the same will happen for the globe as a whole, but not now. We're still hugely dependent on powerful centralized governments