If the shop is given the car and authorization to do something, then they have the responsibility of doing it. That is why they are being paid. They said they can do it, so you gave them the car. If they can't, then they have not earned their money. They are professionals. You are not paying them to experiment or learn thru trial and error. That is something you can do yourself for free.
I would stop them now, before they go any further, and tell them since they appear incapable of doing the job, you will take the car somewhere else.
PS. Unless you live in AZ or south Florida, AC on any early 911 or 912 prior to 1990 is crap. You might be better off putting an open ice chest in the back seat!
