Originally Posted by stpug
Gases will readily mix so the co2 will have just mixed with the oxygen immediately......
I agree with this statement. I believe there is a common misconception of how "purging" oxygen/air from a filled keg actually works.
A shot of CO2 simply mixes with the air (21% O2 / 0.03% CO2), which reduces the oxygen percentage and increases the CO2 percentage within the gas mixture. There would be no stratification of the gases due to the turbulence of injecting the CO2. I would think that one would need to wait a fair amount of time, likely days, for the gases to actually separate out, if they ever did. After all, air doesn't readily separate on its own.
A few pressurize/purge cycles would eventually reduce the percentage of oxygen down to a level that oxidation would be of little concern.