Wow. One in a million? There are other much more likely things to cause that, and to see two switches seem to cause the same symptom seems improbable. I know you have no suspicion of the servicer, but having been there, I can't help but wonder if he flexed the dolby assembly and heard the channels pop back in. I thought I had a bad switch in a 9090 before I knew about through-board connection problems. I did have one bad switch in a 9090 but it was a current-carrying terminal that supplied power to dial lamps only when tuner was selected.
Switches of all types do go bad if they've seen a lot of use though. I've just never used a dolby switch, just leave it off, except to work it around when I operate all switches looking for noisy contacts. Maybe that's the problem, it is a switch that rarely gets used so any oxidation never gets scraped off. You did work all the switches to see if there was any sign of life from the rear channels, right? I think there's also a separate protection relay for Front and Rear but I'd have to dig through a schematic.
I'm interested to hear the outcome.
-john