Freezers are generally preferred for the reasons you mentioned in 1.
2. - I'd steer clear of adding unknown flora to your cave. Unless you know the rocks and such harbor desirable microorganisms, just stick with the ones you add to your cheese. You'll probably find that the cave picks up a variety that is a mix of the cheeses you make.
3 and 6 seem to be direct contradictions. In my experience, it takes some effort to get a fridge up to the desired humidity. If you load up with very wet cheeses you may have a problem going too humid and may need to blow some fresh air in once in a while. Either way, it will take monitoring. I use this
http://www.amazon.com/gp/product/B004I08CEO/ref=oh_details_o03_s00_i00?ie=UTF8&psc=1 4. A commercial fridge/freezer works the same way as a home fridge so an external thermostat should work. The drawback is that until you load it up it is harder to maintain humidity and temperature.
5. Refrigerators and freezers only cool. You'll need to come up with some way to heat them when the temperature around the fridge gets too cool. As long as the cheese doesn't freeze it won't be hurt but aging will slow remarkably. Since many cheeses want to age in 50-55 degrees, whenever the garage temperature drops below that your aging essentially stops. You're better off finding a spot inside. I have two dorm fridges in a closet. The bonus is that although I don't keep my house warm in the winter, the closet is warm enough for the warm aging phase of Swiss styles.