Well, it's fixed. You couldn't tell from looking in the front, but the rear cats were also clogged. They were JUST starting to break up too. Once I took it off and could really look down in there, the driver side was basically totally clogged with debris from the driver side front cat.
Miraculously, and in no way in a manner that violates any federal laws, all of the material came out of the rear cats.
Since all the stuff was already out having fallen out of it's own devices and by no fault of mine, I decided to go ahead and bolt them back up for testing purposes to verify that this really did fix the problem completely. You can see here that they should no longer be obstructed in any way.
Nothing but the O2 to stop the flow of exhaust now.
Then I buttoned everything back up and took it for a drive. Holy crap, it feels like I picked up 100hp. I honestly probably did compared to how it was sunday when I came back from my trip, it effectively only had the passenger side cylinder bank, and even that was partially obstructed. It now goes up hills in top gear effortlessly at 70mph without even needing a downshift and with no check engine light and no fault codes. The exhaust is barely perceptively louder without the rear cats, but definitely still quieter than pretty much any aftermarket muffler.
So in summary, if you have the P0420 and P0430 codes in conjunction with a P0300 random misfire code, there is a VERY good chance that it's not ignition, or fuel injector, or spark plugs or anything related and is solely being caused by excess backpressure generated by your jacked up catalytic converters. Also I can verify that for testing purposes only of course, you totally don't even need the rear cats, it doesn't cause an engine light or anything (which is obvious because there are no O2's after the rear cats to verify their function, but still).
I'm putting all of this, including the trouble codes and stuff, in hopes that someone will find it in a search and spend less time obsessing over every little possible cause, like I did, lol. It's the cats, replace them all.
Thanks guys for all of your help!