The news for the owner is that the temp gauge has been working and the CEL is off. Changing the ECT did not fix it as it still set codes and misbehaved after. The last thing I did was to partially disassemble the female side multi-pin connector on the left front and reseat the sockets as best I could, then reset the codes. If it had been truly open circuit then it should have set a P0118, but perhaps it was a resistive connection.
Anyway, if you run into this common problem (and GM's firmware bug), it might be worth a look at that connector.
I'm done here - I provided much more info than I received.