Try to get you your hands on a volt/ohm meter that you can trust to give you an accurate reading. If the sensors are within specification, look elsewhere as your servo IS operating properly (be thankful for that!) as well as the vacuum elements.
It seems unlikely that your temperature wheel is that far out of sync unless it was moved previously. If the potentiometer were dirty, it would increase resistance resulting in a greater trend towards heating, so again not very likely. The temperature wheel can be moved on the shaft without the special tool -- I happened to have had some very thin wrenches that I think were meant for bicycle hub bearing adjustments (at least I think that's what I used in the past, not sure though). Bear in mind that any adjustments made were minimal, a few degrees at most -- just enough to get the temperature wheel recalibrated to reflect actual cabin temperatures.
I have fixed all sorts of problems with these early climate control systems, including actually rebuilding the servo itself. I hesitate to make a definitive diagnosis, but I would take a good hard look at the amplifier. If you can get your hands on another one to try out temporarily, go for it.