Hello and welcome to just answer. Was the radiator warm at all or cold?
It was cold!
Ok if the heater was hot, but the radiator cold, then that tells me the coolant was not flowing through the radiator, but was flowing through the heater core. That tells me the water pump is working, and the reason coolant was not flowing through the radiator is 1 a stuck closed thermostat, or 2 a plugged radiator. Now you said that the thermostat was replaced so either it is a defective one, or you have a blockage in the radiator. I recommend redoing that thermostat first (maybe your mechanic will do that under warranty). If the problem persists after replacing the thermostat then replace the radiator.
I hope this information helps you, and let me know if you need further assistance with this,
Waaa! I had it done in Algodones Mexico because I had to get home! Also, why would it be a little low on fluid and not "over heat" until I drove - with the AC on - for 250 miles???
Hmmm, It may be an intermittent thermostat issue, or the other thing it could be is a cylinder head gasket that is leaking just a little. This could cause combustion gas's to get into the cooling system and cause flow problems. So, at this point, have a shop do a "block check" with a chemical cooling system block checker. If it fails the test, then the heads will need to be pulled and inspected. Now if it does pass the test, then go with the thermostat/radiator recommendation. One more idea I just thought of: If you find out that there is a leaking head gasket, and you are getting rid of the car in 6 months, try a bottle of Blue Devil head gasket sealer. I have used this product before, and it works. Here is a link their web site.