Code PO420, Service engine light?

While it is possible I have only seen it one time and, in that case, it was obvious because the sensor was sending a signal that was out of the range it should have been. What you want to do is select the data from the bank one sensor two oxygen sensor and compare it to the signal from bank one sensor one. What you don't want to see is them tracking together. The front sensor should show a similar pattern to what you see in the short-term trim you posted. You don't want the rear sensor to look that way. Instead, it should only have minor fluctuations. If it tracks the same, then it is likely the converter is bad. What you could try is to add some Cataclean to the fuel and see if it helps. If it does and the light goes out, you can just watch the data later to see if it has started to go bad again. If the cleaning doesn't work, then you will need to replace the converter. For that it depends on the laws where you are, some states allow a universal type unit to be installed, other states require either a factory OEM unit or a CARB certified part.

Yeah that is showing a bad cat. Notice how the two signals look the same? The one from sensor two should be an almost flat line with just a small amount of ripple.
The most common reason for failure are misfires and excess fuel in the converter(s) actually melting and glazing the catalyst materials, next is contamination from oil or coolant being run through the engine that coat the interior with soot or sludge.
As for what parts you can use, that depends on where you are located. Some states are very harsh when it comes to converter replacement. NY for example requires you to use either a factory replacement unit or one that is CARB certified. The fines for using something else are high enough that you really don't want to push it. NY for example has a cascade style fine, say you take in a vehicle that has a V8 in it and has converters in the manifolds plus two more under the car, so you install headers and a full exhaust system. The fine for the first converter being removed is $2,500.00 but the fine for the next is $10,000.00 And for each violation after that the fine increases by 10K per violation. So you removed 4 converters on that vehicle. That could end up costing you a total of $62,500.00 in fines plus the possibility of jail time. Now sat they find that you have 2 of those vehicles and both have the same treatment, you will be paying fines for a long time.
CA is just as bad, and a few other states have similar laws. The "cheaters" are also illegal, and only work on a marginal converter, not on ones that are completely used up like yours.

A purge solenoid will dump fuel vapor into the engine and cause it to run very rich. The questions to ask about what might have caused failure - Does it use engine oil? Does it use engine coolant? Has the fuel economy dropped off recently? All three of those can contribute to poisoning the converter. Notice that sensor 2 is hitting the same voltages as sensor 1? That shows the converter isn't working. What you would want to see is the front sensor voltage changing as it is, that is because that system controls the fuel by a simple method, it adds fuel until the sensor says it's rich, then it cuts fuel until it reads lean. It would be like you stepping on and releasing the gas pedal 1000 times a second. The average of that is what the fuel trim corrects. The first sensor controls that. Then the exhaust goes through the converter and hits the second sensor. It looks for the same things, is there excess hydrocarbons in the exhaust? Are those in time with what the engine is doing in response to the front sensor? If it is tracking the lean and rich changes then the converter isn't working as it's job is to burn off the exhaust to make it cleaner.
So, you need a new converter that is legal in your state. The "extenders" only work if you have a marginally bad converter, one where it only throws a code once a month or less. They are also illegal in many states as they fall under "tampering with a pollution control device". In this case it isn't going to work, you need a replacement converter. At least this isn't one of the ones that you need to strip half of the vehicle to get to it.
The GM number is - 12568312 GM also shows it as discontinued however there might still be one out there. Or you could use a AP-Eastern 770421 unit if you are in a CARB state or the AP-Eastern 642543 if you are not in a CARB state. Replacement is going to be Disconnect the rear O2 sensor and heat it up to remove it. Then cut the studs off at both ends of the existing unit unless the nuts are in good shape. Remove the unit from the hanger. Install the new one with new gaskets and tighten the nuts to 26 ft lb. Install the O2 sensor and connect it.

If it is burning coolant, yes that can damage the converter. It is the additives in the coolant that hit the ceramic substrate in the converter and the water bakes off and those additives build up and act like a glass coating over the ceramic substrate. That prevents the exhaust gasses from contacting the metals on the substrate and the converter stops working. The question then becomes how much coolant is it using? If it's say a cup a year, that would take a few years to do damage enough to cause a 420 code again. If it's a gallon every couple months then it would likely damage a new unit in a few weeks.

Pulling the sensor won't test it. The P0420 code and the changing voltage already shows the cat is bad. The video you posted confirmed that. If it was excess fuel from the purge valve alone it has burnt off by now because it would have been a constant vapor source that is now gone. The canister won't be saturated as the valve draws the vapor out of it. Now that it is operating the system will return to "normal".

You are welcome. Let us know what you decide and how it turns out. Might help someone else who has the same problem.

It usually takes a bit for a P0420 to set as there are other items that have to happen before it will run the tests. The EVAP leak shouldn't be an issue in that test. I would try the Cataclean and see if the secondary O2 calms down. I sort of doubt it but it shouldn't hurt it.
I just replaced both of the failed converters on a Chrysler at $1,200.00 each. This state requires CARB or OEM only so you pay through the nose.

That temperature is very low for a correctly working cat. That would be one item that might give you a hint if the Cataclean worked, if the temperature goes up after the treatment it likely cleaned out some of the crud and could give you some extra time. Yes on the flashing CEL, that is generally telling you that something is trying to wipe out the cat.

If they will actually fill that order at that price and it's legal in your state, then I would order 2 and put one up for sale to get your money back.

Still real low, a working cat should be up around 800 - 1100 degrees depending on engine load.