I was having trouble getting the 210° On, 195° Off switch hot enough to trigger in a simple boiling water bath, so I went back to the bike. I attached the wire from the wiring loom to the switch terminal and grounded the body of the switch to the frame of the bike by running a heavy-duty jumper cable from there to the bare-metal passenger peg bracket.
I proceeded to gently warm the switch (now hanging in mid air) by waving a very low flame from a propane torch onto it while checking its temperature every few seconds with the non-contact thermometer. I couldn't check the temp with any real precision that way, but I got it between 210° and say, 250° at most. When tested this way, BOTH OLD AND NEW SWITCHES WORKED! The relay clicked and the fan came on. Letting the switch cool opened the circuit and the fan shut off.
A professional mechanic mate of mine suggested that the relay might have been the real source of the problem all along as in its 41-year life it had gotten "lazy" from dirt or simple lack of use (the bike only has 6000 miles on it). That feels plausible to me, and I'm hoping that all the extremely rapid open/close cycling of the relay hundreds of times during testing has shaken it out of its torpor for now.
Today I'll make sure the coolant reservoir is topped up, then run the bike up to temperature and see what happens!
If the fan was "intermittent" I'd say sure the relay could be at fault. You could pop the cover off of the relay and inspect/ test/ clean the contacts. Relays tend to arc and corrode the contacts. You could energize the relay and check the resistance across the power terminals. You could also just replace the relay with the common 4 pin 10 A relay for $10 (Bosch)
Yeah, and I've already got a half-dozen of those cube relays lying around, most of which came with sockets and a pigtail... If the thing still doesn't work this time, I'll at least make an adapter to test the theory.