Thanks for the response. I was checking voltage at the single (hot) lead into the compressor. I wasn't able to safely check it at the clutch pulley. In hindsight I should have checked resistance from the compressor block to the engine block since there is no (-) wire out. It a great tip for future diagnosing.
I was puzzled about the low pressure and why the compressor would not turn on. I could not get the pressure any higher, though, so I had to assume something electrical was going on (or not going "on") inside the compressor.
Desperate for a/c and for lack of being able to get more than 20 lbs into the system I chanced a $140 junkyard compressor. After swapping, it charged perfectly and blows super cold. I'm still curious if the compressor has an internal pressure sensor but I'm pretty sure now that it was just the clutch. Maybe a bench test will confirm.
Lesson learned: Never assume the obvious problem is the ONLY or most serious problem. So take lots of measurements BEFORE starting, even if doesn't seem relevant to the repair.