One of the first questions in this article:

"How does a very dense gas like R-12 get elevated to 5 miles above the earth?"

Note: Freon 12 is about four times denser than oxygen, and three times denser than carbon dioxide.

First point: atmospherical circulation.
The atmosphere is not static; apart from the horizontal motion we feel as wind, there is vertical circulation driven by temperature differences. Soaring birds and glider pilots use small scale 'thermals' over warmer patches of ground to gain altitude. Larger patterns have air rising above the tropics and descending again near the poles, the motion complicated by the rotation of the Earth.

Even if these air movements were not enough, there is another phenomenon that needs to be considered: diffusion. This is the process which causes a mixture of gases to tend to become evenly mixed over time, due to the random motion of the molecules. The effect of gravity in causing heavier molecules to tend to lower layers and light molecules to drift up is negligible because of the velocities of the molecules - of the order of 1000mph - and the short distances they travel before colliding with other molecules - about 60 trillionths of a meter at sea-level to about one-tenth of a meter at 100km altitude. Also note that the speed is lower for heavier molecules, but as the square-root, ie a molecule 100 times heavier will have an average velocity only one tenth that of a lighter molecule. So Freon 12 will diffuse at about 60% the speed of carbon dioxide, not such a big difference.

If the argument was valid, then our atmosphere would tend to be stratified into layers of different density gases, with all the heavier molecules, like carbon dioxide and most pollutant molecules close to the surface, with water vapour at the top as the lightest common molecule in the atmosphere. This doesn't seem to be what we experience...

Regarding the release of chlorine from swimming pools, etc, I think the problem is the these are highly reactive molecules, unlike CFC's, so they don't persist in the atmosphere long enough to reach the stratosphere, where most of the ozone creation/depletion action is happening.

THe effect of volcanoes is short-lived, again because most of them don't deliver the chlorine high enough to reach the stratosphere directly. The chemistry is complicated, but under some conditions of low background atmospheric chlorine, volcanic eruptions may actually increase ozone concentrations, because some of the nitrogen compounds that are also often released actually disrupt the ozone-destroying reactions. See

http://www.aer.com/scienceResearch/volcano.html

Another question:

"Why is the ozone hole over Antarctica and not over the USA?"

The process is enhanced by the polar stratospheric clouds which can only form under polar conditions and which provide particles on which the various reacting compounds collect and are brought together to greatly enhance the particular reactions. In addition, the atmospheric circulation (the Antarctic Vortex) around the pole tends to limit mixing with the rest of the atmosphere, thus keeping the nitrogen compounds which are generated in mid-latitudes from disrupting the process, which also helps explain why there is no major ozone hole over the USA.

I don't think there is any evidence that volcanic activity in Antarctica is more intense than in many other parts of the world.

The process almost certainly occurs to some extent over the whole globe, but conditions are more favourable over the polar regions and Antarctica in particular. As concentrations rise, the effect will increase. Even if the major action happens over the poles, seasonal changes in atmospheric circulation distribute the ozone depleted air into mid-latitudes, so it will eventually start to affect places like the USA. We in Australia are already at risk because the Southern hole is much more severe.

A good detailed discussion can be found here:

http://www.ccpo.odu.edu/SEES/ozone/oz_class.htm