Your vacuum leak is probably one of the diaphrams under the dash. It's very seldom a valve, they only open or close. If one of the diaphrams have dry rotted, it will leak. Some of them are a booger to get to. I've had this problem and the first one I'd replace is the one for the center flaps. It's to the left of the glove box (at least on my 85 300SD).

Monovalve: when they stop they will always stop open allowing heated water in all the time. It should get hotter w/ faster RPMs.

What does your engine temp get to? It could be a stuck thermostat. It may not be closing enough to keep the coolant heated. It's cooling off too much with air flow but heats up sitting still.

As far as your A/C: Never heard of this problem. Usually the slower the RPMs the less cool the air. Unless it is a problem with the Climate Control Unit not opening the correct flaps properly?

The aux water pump: If it fails you will have less heat at idle. It increases coolant flow at low RPMs. HOWEVER, when they build up enough resistance in the motor it WILL burn out your Climate Control Unit. It can be repaired with a wire soildered in the right place.