Putting a spark plug non fouler on the rear O2 sensor is not fixing the problem, it is tricking the computer by giving it false readings. There are test that can be done with a scan tool to find what the problem really is. It is possible the front or rear O2 sensor is not reading correctly causing false reading to the computer. I don't know if the dealer put a factory converter or an aftermarket one on, but there is a huge difference in them. That is why there is such a big price difference with them. A catalytic converter works because it has precious metals inside it. The aftermarket ones do not have as much of those precious metal in them so they do not work as well. Some of the aftermarket ones will not work on OBDII vehicles.
If it has only been a couple of hundred miles since the dealer diagnosed it I would take it back and talk to the service manager. I worked at 2 different GM dealers for a total of 9 years I know how they operate. It makes a huge difference which tech gets the job. There are good and bad technicians in just about every shop out there including dealerships. The service manager should get involved if they have had 2 attempts and the problem is not fixed. I have never seen a computer be the cause of a P0420 code. I have seen software updates for a P0420. You can go to this website and see what the current software is and any updates for your vehicle by entering the VIN. https://tis2web.service.gm.com/tis2web. There are no TSBs for any issues with a P0420 code on your car, but that does not mean there is not a software update.
If the dealer still says it is the computer I would request that they call GM's TAC (Technical Assistance Hotline) to see if they agree with the dealers diagnosis. I would be VERY surprised if the PCM is the cause of this failure. I would guess at least one of the O2 sensors was defective
