People just add flags in their mining protocol that only trigger when a threshold is reached. The last Segwit changes needed a percent change of closer to 90% just to trigger the next change.
We are only assuming that consensus would be reached quickly given the scenario presented. It would be irresponsible to design it to need simultaneous action. People would have to considering to stop using the bitcoin network for X,000 blocks while consensus is being reached, and only until it is reached.
Anyone can make a snapshot, assigning value to it is not centralized