diffusion will destroy the density gradient
Bill Tindall, E.Tn.
"Argon introduced into a space can form a layer below the air"
Indeed it will, temporarily, because of density. But this density gradient is quickly destroyed by diffusion. The gas molecules are in rapid motion. This motion is what exerts pressure on a container. The more gas the more the pressure. This motion rapidly moves gas molecules within a container. After a short time any gas or vapor in the head space will equilibrate to a uniform mixture. An average molecule of argon will diffuse (move) 1 cm in 2 seconds.
You can illustrate how fast gases move with the following experiment. Put a drop of ammonia on some cotton and a drop of hydrochloric acid on another. Put one cotton ball on one end of a foot long tube and the other cotton ball on the other. When, and where, the hydrogen chloride meets the ammonia a white ring of ammonium chloride will form.....within minutes. Ammonia, because it is smaller molecular weight, will meet the hydrogen chloride closer to the hydrogen chloride end.
A more country experiment is to provoke a skunk on a still night and see how long it takes the result to reach the front porch, or bedroom window if it is a summer experiment.
It is a good science demo in you happen to need one.