From a pure physics perspective "tank rolling" to avoid gas stratification is clearly total bunkum!
Gases are driven by Brownian motion, the vibration and and random movement of their molecules in that gas. This movement is what gives us "pressure" because that pressure is actually those molecules hitting and bouncing off the walls of the tank. The more you compress something, the higher the pressure, because in any given volume there are more molecules hitting the confining walls at any given moment.
So what would be the mechanism for stratification of gases in a tank? Clearly the main one, assuming there is no direct chemical reaction, is gravity seperation due to the differences in gas density.
Here are the densities for these gases at 1bar 0degC
Oxygen: 1.43 g/l
Nitrogen: 1.25 g/l
Therefore oxygen is just 14% more dense, ie it experiences a 14% greater force in the direction of gravity.
This is clearly a small difference and driven only by the force of gravity and means that brownian motion is many time more dominant even at 1bar, and certainly way too small an effect to overcome the forces of brownian motion at hundreds of bars of pressure.
Industrial gas centrifuges that are expressely designed to seperate different densities of gas require high speed rotation that acts with tens of thousands of times the force of gravity in order to practically seperate gases with such small density differences.
And of course, rolling a tank itself doesn't mean the gas inside actually moves, as that gas has an inertia and only the small amount of boundary layer friction between the gas and the moving tank walls would drive the gas around with the tank.
Finally, the hole that routes the gas into a cylinder is pretty small, a cross section in the order of tens of square millimeters at most, which means the incomming gasstream velocity is pretty high, (which is also why you fill tanks slowly to avoid dynamic high pressure, and hence high temperature, zones) and that high velocity stream causes massive swirling and mixing of the gases inside the cylinder, once again rendering "rolling the tank" as a total waste of time.
So, we are left with the conclusion:
1) tank rolling doesn't cause the contents to mix
2) The contents mix themselves via brownian motion
3) the force of gravity is insufficient over any time scale to cause stratification due to different constituent densities
Finally, with regard to breathing pure nitrogen, ie at a PPo2 of zero bar. The old airforce tables give an expected time to loss of conciousness of as little as 5 seconds at pressures below 0.05 bar. At a true zero PPo2, your lungs will actually REMOVE the existing o2 from your bloodstream. Chances are, i'm going to suggest that a single full breath of pure N2, is likely to result in pretty much instaneous LoC, and assuming the next breath wasn't suitably hyperoxic death within a minute.
