If the evaporator case that you're talking about is the tray that fits underneath the dashboard, under the glove compartment, (12 inches x 18 inches, by about an inch high, [my estimates]), that tray has been replaced and is new. As I said, the Buick dealer has sealed this case a number of times, and there are channels within the case specifically for sealant. These channels hold a soft but semi-rigid sealer that fits in these channels and compresses in both surfaces as the tray is put into place. The drain is through the firewall, roughly in line with the glove compartment, and condensation leaves huge puddles of water on the ground as it drips to the ground through this hole. This drain appears fully functional.
You say "Condensation can form when there is an air leak in the evaporator case." There is no question that condensation is forming. That happens by virtue of an operating air conditioner, and the function of this tray, I am told, is to handle this condensation which is normal when the AC is operating. I have also been told by the dealer that it acts as if the air flow is being restricted in some way, but they assure me that such is not the case.
I was hoping that there was some unknown, little detail that the mechanics and body shop personnel are overlooking. Some small trick or something that is being overlooked that is peculiar to a 1992 Buick Roadmaster AC system.
The Buick dealer is Rahal Buick in Marianna, FL, and they have been in business there over 60 years. They are quite familiar with Buicks. They are excellent people and I don't know where you could find any better, but this one has them stumped.