The Temp Switch/Sensor for the fans on the Radiator is wired separately from the AC wiring!
There is an Ambient Air Temp sensor. It is located on the left side behind the cowl just below the corner of the windshield (on A4 Jettas for sure). If critters chew one or both of the wires, AC will not function (don't ask me how I know).
Yes, the coil and clutch can be replaced without dumping the refrigerant.
Yes, the pressure switch will not allow the Clutch on the Compressor to kick-in if the refrigerant pressure is too low.
I've been running straight 12 volts on the AC coil with the ALH in my Vanagon since August, 2012.
Fuse #16 is for the AC compressor clutch.
Yes, check voltage at the compressor.
Yes, check the ohms resistance of the clutch coil.
You need a set of Manifold Gauges if you are going to do diagnostics on the AC system.
Typically, the amount of refrigerant (R134a) is labeled somewhere under the hood. It may show as much as a 10% deviation (plus or minus 5%). Thus, you do not have to be "perfect" in charging it. R134a goes in slow. Ambient temp, max AC setting vs normal, out front fans, etc., all have an affect on the pressures you will see on both low and high sides. Switching from Max to normal will show an increase in both low side and high side pressures (all contingent upon temperatures). But, generally about 35 psi to 45 psi low side and 150 to 200 psi high side is what you need. Always check dash vent temps as you add refrigerant... something in the low 40s is what you want. Again, ambient temp, direct sunshine, etc., all affect pressures, temp, etc.
Hope this helps!
EDIT: In reference to the work order. In all the work experience with VW AC, research, studying wiring schematics, etc., I've never ran across anything that indicates that the Fan Control Module "senses" anything! In my opinion, that statement is absurd!