"NOW, here's what I don't understand: when I disconnect the red and white (heater) wires, the idle surge goes away and the pulse flatlines at 0.0 volts and the engine runs smooth as silk."

Disconnecting the red and white wire connector also opens sensoer the ground side, resulting in no signal voltage to the ECU. The ECU should set the CEL and a 2-2-1 code if no O2 signal is detected...
"...for more than 3 minutes under constant load, for example during idling...".
See if you get that ECU response after 3 minutes of that "smooth as silk" idle.

The smooth running is perhaps from to a rich mixture due to a no-signal default ECU reaction.

The sensor voltage swing is within limits, but I can't explain the slowness.
I don't recall any bad Titania reports here in 6+ years, but who knows?

The heater circuit doesn't turn off. It's powered from the Fuel relay.

--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.