So I understand what you are trying to say, it looks like you are basing your recommendation on the premise that (A) the cat is reliable and likely to be good and (B) that a bad O2 sensor will give a false P0420 or P0430.
Regarding (A), one of the potential sources of catalyst contamination I have I have not mentioned before is the particulates in the exhaust stream. If you wipe the inside of the tailpipe with a white rag, it will come out black with soot. While most of this partiulate matter gets burned off in the catalytic converter, some of it does not and the catalyst eventually becomes coated with the particulate. Getting the exhaust good and hot will delay the onset of this condition, but not everyone drives the car hard enough to do this. If the catalyst is coated with particuclate matter, it cannot act on the exhaust gas, and the cat loses effectiveness and the ECU will put up a P0420 or P0430 code. A contaminated catalytic converter is not as uncommon as you think.
Regarding (B), there are 4 possible scenarios. For these 4 scenarios, when we say the downstream #2 O2 sensor is "bad," we will assume that it is not bad enough to give its own DTC and is operating marginally or the sensor head is contaminated. If the cat is bad, the gas will come out of the cat in the same condition it went in, so #2 sensor sees the same signal as #1 sensor and if they are working correctly, they will send the same signal to the ECU. The ECU will set DTC P0420 or P0430 if it detects the same fluctuating signal from sensor #2 as it is getting from sensor #1 over 2 trip cycles. The ECU will not set a code if the signal from sensor #2 is flat.
1) If the cat is good and #2 O2 sensor is good, #2 sensor signal is pretty flat and the ECU does not set a trouble code. 2) If, as you assert, the cat is good and #2 sensor is bad or marginal, then the sensor signal would be pretty flat - so to the ECU, the signal in Scenario 2) looks the same as the signal in Scenario 1) so the ECU would not set a trouble code 3) If the cat is bad and #2 sensor is good, then the signal from the O2 sensor starts fluctuating and the ECU sets DTC P0420. 4) If the cat is bad and #2 sensor is bad, the signal would be flat and the ECU would not set a trouble code.The ECU has to see activity from sensor #2 to set P0420 or P0430, and the only scenario wher it would see activity is Scenario 3.
I agree that the cat is more reliable than an O2 sensor, but you are totally missing the conditions necessary to set DTC P0420 or P0430.
There has to be a signal from #2 sensor to set those trouble codes, and a bad sensor will not send a signal, so a bad sensor cannot set those codes.
Also, although the cat is more reliable than the sensor, they will lose effectiveness over time and can eventually go bad.
My experience is a little less limited. When I have encountered P0420, the cat was bad.
As I have mentioned several times, a contaminated cat will not cause driveability problems. I do not know where you got that idea, but it is totally incorrect.
You know what happens when you make assumptions! ;-)