PGA is Pure Grain Alcohol, also known as Everclear, and is drinkable. Isopropyl alcohol, on the other hand, can be toxic to the human body and is not recommended as a cleaner or thinner for any of our e-cig equipment.
If you can't find any PGA, then the highest alcohol content Vodka will work just as well.
I prefer to soak for at least an hour. Usually I leave them soaking over night, though that long isn't necessary.
I use the alcohol simply to displace the water left inside the atomizer. Water (except distilled water) has impurities that can build up on the coil over time. Just look inside your water pipe sometime and you'll see the calcium buildup.
Alcohol leaves no residue to coat the coil.