AC Problems & Melted Overflow Tank

We have a well maintained 1999 Acura TL with just over 128,000 miles. On 6/10 all the indicator lights were lighting up, the car would not stop running after the key was removed and the alarm would go off for no reason. On 6/11 the car would not start at all. AAA came and replaced the 6 yr. old battery. On 6/12 we experienced some of the same problems with all the indicator lights lighting up so we drove it to our mechanic. On 6/13 our mechanic had to charge the battery because it was totally drained. Since then, and up until 6/29, all was ok. Now the real problem: AC would crank cold then stop working. The next day, after approx. 15 mins. of local driving the AC stopped working, then we started to hear a rattling noise, there was a burning smell followed by a lot of smoke from under the hood. The car then stalled when we were forced to stop. The fan on the driver's side would not stop running after the car was turned off. Of course this was early in the morning after a freak storm (a Derecho) that downed trees & power lines, with widespread power outages. Did I mention the 90+ degrees & high humidity! Anyway, on 6/30 (Sat) we had the car towed to our mechanic. On Monday, they found "the thermostat stuck & burnt vent valve & melted overflow tank". They replaced the radiator cap & thermostat, the vent valve transfer (used) and added anti-freeze. They also epoxied the overflow tank in places where it had melted through. (So we would have the car for July 4th, and since they weren't positive that replacing the overflow tank with a new one would fix the problem, and it would need to be ordered anyway, they epoxied the old one). On July 4, we drove the car approx. 40, mostly highway miles, with no problems. On our return home, we drove approx. 30 highway miles with no problems. After about 5 city miles we made a stop, then it all started happening again. The AC stopped working. By the time we got home it was starting to smoke, etc. After a "rest" we drove it a few miles to leave it at the mechanic. On 7/5 they found another small leak, which they epoxied. We were told to take it out for a highway run because it seemed that the problems started after fast, highway driving. For 2 days we drove the car maybe 10 - 15 miles at a time, no more than 25 mph, with no problems. On 7/8 (today, Sunday) we started our highway test drive. About 15 mins into it, with speeds no more than 45 mph for maybe 5 mins. the problems started all over again. The AC stopped working so we turned it off, turned around, and headed home. Approx. 10 mins. later we tried the AC. It initially worked, then stopped. After a 6 hour "rest" we decided to drop it off at the mechanic to be looked at the next day. The AC worked for about 15 mins, then it all went wrong, again! AC stopped working, loud rattling noise from underneath, slight burning smell and hints of smoke. Eventually the car stalled at a red light. A check under the hood showed that the overflow tank was empty and the driver's side fan was still running, even though the car was turned off. We filled the overflow with water (not sure if that was the right thing to do). 15 minutes later the fan was still running. I looked underneath and something was dripping on the passenger side (this is the first time I thought to look underneath). It felt slightly sticky and had a slight sweet taste - yes, I tasted it! I wanted to see if it was water because I'm sure we over-filled the overflow tank. Anyway, after a 1/2 hour rest, the fan was still running, but we were finally able to start the car and get it to the mechanic. So that's our tale of woe. Do you have any suggestions? Is it coincidence that all these problems started when the temperatures soared well into the 90's? Was that anti-freeze I tasted? We have great trust in our mechanic and he has decades of experience, but I'm afraid that this one may have him stumped. We love our Acura and are hoping that it can be fixed without costing thousands of dollars!!!!