Steve - My planned approach for Friday is to do exactly what you said. Will test the mono-valve circuitry. In looking at the schematic this evening I noted that it gets its positive directly from the fuse - would be nice if the problem was as simple as the fuse being blown or corroded. Often these complex & frustrating problems end up being so simple as a fuse - that should have been the starting place on the task.
Do you know the pin numbers that can be jumped on the A/C relay on the other vehicle I mentioned in the query?