CO2 is indeed lighter than air, but when you pop the lid off it creates turbulence and a pressure differential, displacing the CO2 blanket.
OK, anything is possible! - BUT - Have you ever oxygenated beer to the point you tasted cardboard? Time to set up on a soup box! :rockin:
OK, I am the last to tempt the "....'s" but come on.
In reality, that does not tend to happen unless your using a very flat fermenter, and open it in a hurricane that mixes the beer up to oxygenate it.
Maybe if you pour from a carboy, into another carboy 2-20 feet away with a cardboard funnel splashing everything around everywhere you will be able to taste some cardboard tasting oxidization. But if your doing that, I'm pretty sure you have bigger problems and will not notice the off taste.
We are talking about dry hopping here, and when to do it, not yet another homebrewers boggy man.
To the OP, I again recommend in the primary after all (active) fermentation has stopped and leaving it in for 3 days to how every many weeks. (I do this with pellets, so they settle).
For spices, or fresh whole hops, secondary is fine, but again wait long enough for the full impact before racking/bottling/kegging.
--- Stepping off of soup box, grabbing another homebrew and logging out. ***