I tried a few more things tonight. First, I found a massive vacuum leak. I can't believe I overlooked this prior, but the intake tube was not cleanly seated to the throttle body (its an aftermarket "piece)". I fixed that and cleaned the MAF while it was apart. While it helped the idle and cold stumble, it didn't solve the larger issue.
I swapped out the 02 sensor that was causing the P0136 code. I am still not getting any voltage reading out of it, and the code came back. I am going to assume there is a problem with the harness? The prior owner did replace the TC, so the transmission has been out of the car or at least moved, so I guess its possible they pinched a wire.
P0172 and P0175 quickly returned. Is it reasonable that both banks would be rich due to one downstream 02 not having a reading? It seems like more than an 02 issue at this point.
I monitored the fuel trim readings, and they seem off to me, but I really don't know what "normal" adjustments are. The Short Term % on either bank at cold idle was around -13 or so; they improved a bit once warmed up, but still ranged anywhere from -1 to -9 and bounced between that range continually. While driving, they were all over the place, anywhere from -28 to +41 (WOT run). At one point during normal driving they bounced from the - single digits up to about +20 and flat lined there for a 5 seconds or so, before dropping back down to the negative side. It just doesn't seem like normal behavior to me.
The Long Term Fuel Trim once warmed up was fairly consistent at -20% on both banks. That seems like a lot of compensation for me, but again, I don't claim to full understand how this all works.
EDIT: I read this article
and it explains this fairly well, yet leaves me without a solution. So the LTFT being at -20% on both sides confirms a legitimate, ongoing rich condition. Also (according to this article), the downstream 02 sensors don't play a role in fuel trim adjustments, so the P0136 seems to be isolated to itself. In addition, a vacuum leak would cause a lean condition, not a rich one, so while it is good to fix the vacuum leak, it likely wasn't causing this problem.
Going through his basic troubleshooting list:
Ignition System Issues
BAD spark plugs - New within last 500 miles
BAD spark plug wires (4.6L equipped with coil packs) - New within last 500 miles
BAD coil pack (4.6L equipped with coil packs) - Do these ever actually go bad?
BAD spark plug boots - New within last 500 miles
BAD Coil-on-Plug (COP) ignition coils - Doesn't Apply
Fuel System Issues
Dirty or Clogged Fuel Injector(s) - maybe, but on both banks?
Leaking fuel injector - maybe, but on both banks?
Leaking fuel pressure regulator - Pressure didn't seem to bleed down much at all?
Failed pressure regulator causing excessive fuel pressure. - Readings within reasonable range
Air Filter Assembly Issues
If the air filter canister assembly has been replaced - Factory Box
Air filter canister is not latched correctly. - Latched Tightly
Air filter canister has missing pieces. - No
Wrong air filter has been installed - K&N Panel for this Application
Cooling System Issues
The thermostat is missing - No
Incorrect temperature thermostat is being used - 180* Thermostat accounted for in tune; PID shows 190* actual operating temperature
engine coolant temperature (ECT) sensor is not reporting the correct temperature. - Reads a constant 190* once up to temp
I guess I'll test my MAF
next. It seems to be getting voltage as I can watch the lb/hr reading on my scan tool, but I can't view voltage with it, so I'll break out the multi-meter tomorrow and see what I come up with.