A little cutting and soldering later and I think what I saw was that the ECT doesn't plug directly into the ECM.
Here's what I had:
had a false positive, engine overheating condition (engine was cold but ECM reported hot) engine running but not right due to unrelated vacuum leak.
by wire colors, determined (incorrectly???) voltage is sent from ECM to sensor then back to ECM for processing.
found 5v on one wire terminal unplugged, key pos2.
found other wire uh.... 400ohm resistance from terminal to ECM (incorrect?) key off
found 150mv on same wire, key off.
Determined short in the system, cut existing wire and soldered jumper wire from ECT connection terminal directly to ECM wire at harness.
Crank no start condition.
Returned wires to previous factory condition wiring and back to incorrect overheat condition.
(while moving wires back, I read 7v output from ECT.)
Pulled MAF sensor, restart, overheat condition continues for 30 seconds, temp gauge goes dead, radiator fan turns on.
Plug MAF back in, restart, back to crank no start condition, overheat condition possibly cleared, can't remember.
Remove positive battery terminal for 5 minutes, temp gauge now reading correct, car is starting again, and I can get to my next problem of vacuum leak.