Will and Rat -
Bad news and good news.
First, I purchased a new Coolant Sensor. Before I installed it, I tried the boiling water experiment, to see if I could get the low speed fan to come on. The wiring harness doesn't provide much slack, but I was able to squeeze a cup of boiling water in there, under the throttle and vacuum hoses, and I dunked the new Coolant Sensor for awhile. No joy... couldn't get the fan to come on. Which made me worry that there could still be something wrong upstream.
So, what the heck... I just replaced the Coolant Sensor, anyway. Guess what... it worked!
Temp cycles up to almost 220, low speed fan(*) come on, temp falls back to 200, fans drop out. And the low speed is on both fans, not just the drivers side fan. So, maybe the fan relays are wired so that one relay is low speed (both fans) and the other relay is high speed (both fans)... not sure about that, but I wouldn't be surprised.
Anyway, problem solved... thanks for all the help.
BTW - I'm still planning on a 180 thermostat, but will probably wait until spring.