There should be no problem mixing Walker and Bosch sensors as you have done.
But I will point out that the P0161 descriptions in the RAVE are "O2 sensor heater circuit malfunction (bank 2, sensor 2)" and "Rear sensor heater RH bank - short/open circuit." There is no reference to "signal below minimum" as you state, and it has nothing to do with the engine running lean.
Remember, the sole purpose of the rear, position 2, downstream O2 sensors is to monitor catalytic converter effectiveness. Ideally the voltage reading should remain very close to 450 mV, fluctuating more slowly and over a more narrow range than the upstream sensors.
I don't see an UltraGauge or something similar in your long list of modifications. Are you able to monitor O2 sensor voltages and other data while driving?
'99 DII, STI wires, Champion 7070s, '04 CDL & SLABS, Garvin WILDERNESS rack, painted headliner, air springs victim, headgaskets at 109,000, plus victim of XYZ (twice) and many other ridiculous DII failures. Option B AMIGO FREE since February 2011.
Runs like a top; now if I could just afford the gas! December 2014 update - Wait, I can afford the gas! Drill baby, drill!!!