Code P0420, where is Bank 1 Sensor 2 located?

It's highly unlikely that it's a sensor but on that engine, you have the upstream sensors that are in the exhaust manifolds, then the converters that bolt to the manifolds. The downstream sensors are the ones that are in the sides of the converters. For the locations I attached the wiring and locations. The B1 S2 sensor is the one on the side of the converter on the right side of the engine with the orange circle, which would be the rear bank next to the firewall. However, given the cost of a replacement converter you will want to test the sensor(s) first. To do that you connect up the scan tool and watch the voltage that the O2 sensors are showing when you compare the upstream and downstream sensors show. A P0420 code generally means that the downstream sensor's voltage is following the upstream sensor. That would show on the scan tool as the voltages rising and falling together when a good converter would show the upstream sensor changing a lot while the downstream sensor barely changes. Easy to see with a scan tool.

It shouldn't as long as the plugs were the correct ones. A bad plug that is causing a misfire could though. Is the 420 code the only code? What was the reason for the plug change? Misfire or other problem?

Okay, the likely hood is that the misfire dumped raw fuel into the exhaust and damaged the converter. Not an uncommon thing unfortunately. What I would suggest is to go out and run it hard a bit and see if it stays, if so look on YouTube for "anti-fouler and P0420 codes". It's not really a legal repair but it sometimes works. Oh, FYI if you wanted to test the sensor before just buying a new one, the downstream sensors are the same on both sides of the engine. So, if you swapped them and the 420 was replaced by a 430 code (same meaning but for the other side of the engine) you would know it's the sensor. If it stays as a 420 then you know it's the converter.

I have never seen anything that actually cleaned a converter. What usually happens is that oil or fuel will get into the converter and either bake over the substrate or it will melt the metals used and cause the unit to stop working. In either case nothing is going to clear those issues. Where cleaning could work would be if you discovered it was using oil very early and it hadn't cooked on yet. Even then it would not be really likely. Especially with modern honeycomb units like yours. If you look at the design those are really bad as they are a split unit with the O2 sensor stuck in the middle. So, in effect it doesn't test the entire unit, it only tests the section of the converter above the sensor. I've been tempted to take a split unit and add a second sensor bung at the outlet of the unit. Then see if it now tests the complete unit, including that lower half that normally isn't in use. I just need to find one that I can play with.

I have seen videos on there showing perpetual motion, free energy, and how to make millions by helping out a prince. Doesn't mean they are actually true. As I said I have never seen that method actually work and I know folks who have tried it after watching folks like Scotty say it works. The problem is that it might work if the converter is only coated with loose soot or if it's from fuel saturation, but most are not in those categories, usually the contaminate is hard baked on like a nonstick coating or if there was the right mix of fuel in that they will have physical damage from the substrate overheating and melting. You can check the one on yours easily enough. The converter unbolts from the manifold, and you can look at the top layer right there in front of you. That is how you replace them on that engine.
The sensor goes in the side as shown, the substrate is at the top in the opening.