What I am trying to establish, is if the catalytic converter is indeed the culprit, or a lazy/rogue O2 sensor is giving false readings. You are exactly right, the downstream O2 monitors catalyst efficiency... that's why I want to verify that the sensor is working properly.
I understand that more than likely, it's a bad cat... but I just want confirmation that it's not an O2 issue before I spend the money replacing the cat.