Ever touch the compressor while it's running? It's hotter than any exhaust pipe you'll brush against. Statistically, the o-rings at the compressor are the ones most likely to fail due to the heat. May I suggest you replace those (yes, remove the hoses, replace the little green o-rings, re-attach hoses, don't evacuate, just charge with a can of r-134a) and see what happens? If it holds the charge and blows OK cold, you have your answer. Then you could go to the AC shop, have them evacuate it and replace the r-134a and you would be fine.
R-134a is about three times the density of air, so assuming there is no residual pressure in your system, pretend it's full of water. Hold the tubes "up" and/or be ready to put a plug over the open ends and you won't have air entering the system. The issue of moisture entering the system when you are not removing/replacing a component is minimal and that's what the dryer is for. I've done this on my previous Viper and an Intrepid and it worked to perfection. Certainly the cost of o-rings and two cans of r-134a is cheap compared to the original estimate.