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Progress: I swapped back in the O2 sensor that had the 12 ohm resistance, rather than the 3.1 which is spec. The voltage swings on both front sensors came back before the engine was even hot, fuel trims kicked in, and the loop is working. CEL came on after a few drive cycles, and lo and behold we got a resistance limit code for the front sensors (the heaters are on the same circuit). So, either the sensor plug or the sensor wiring or the (new) sensor itself that I removed is bad (though the heater resistance is close enough at 3.5 ohms).
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