After talking with our graduate Associate Winemaker, (degree in food science) and what I have been taught from the owner, also a graduate with specializing in chemistry, I will post the following:
Gases mix = True
Gases stratify = True.
Both are true if you understand that Carbon Dioxide is a dense molecule and settles to the very bottom in a mixed gas enclosure. We gas the tanks at the winery, and have done so since I've worked there. This is from the boss who owns the winery and has the degree. CO2 is heavier than air. It does stratify, but stratifies to be the very bottom of whatever gas (air) it's around. Nitrogen is a gas, but nearly the same molecular density as normal air, so it mixes very well and does not, for practical purposes, stratify or separate out.
CO2 does settle to the bottom and lays in the bottom and as wine is pushed into the tank from the bottom valve moves up the tank until is is purged out the top of the tank through the open lid.
This is what I do, and will continue to do so, and no cutting pipes.... I open the main lid and use pure CO2 and put the tube into the very bottom of the keg, THROUGH THE TOP LID, and then slowly add a good measure of CO2. While it is filling there is mixing, but when the gas stops coming in, the stratification begins.
You can pour out CO2 just like water. I have witnessed this first hand when I've opened the door on a large tank I'm centrifuging from, after the wine level is below the door, and see, yes see, the CO2 pouring out. It does not have a color but you can see the shimmer of light being bent - distorted by the out-flowing CO2 from the tank.
Once I, (or you) have put CO2 into a keg through the top lid opening, allow just a few seconds and use a barbecue lighter and light it an put in the keg. If the flame goes out within 2 inches of inserting into the keg, you have sufficient CO2, not wasted and now you can put on your lid. When ready to fill, generally within a few minutes, you can either take off the lid and put the hose into the bottom of the keg and fill from the bottom up as the CO2 "pours" out the top of the opening, or hook up to the "OUT" side and fill from the bottom - up - by having the pressure release valve in the "HOLD OPEN" position. I prefer the filling through the top of the lid as I can see and stop the flow when the beer is near the top. Immediately upon filling and taking out the hose (if that is what you used) put the lid on and while the release valve is still opened add CO2 for about 5 seconds and ALL of the last of any air will not be purged out and replaced by CO2.
Wine making and beer making is not rocket science. Keep it simple. RDWHAHB
Do this experiment... take a nice clean keg 2.5, 3, 5, size doesn't matter, and put in CO2 for about 30 seconds as I have said above. let it settle and light a candle. Now take that keg and put out the candle by "pouring" out the CO2 form the keg over the candle. Candle will go out, guaranteed.