Big news! At least for the present, the cat problem is solved…and for roughly $20! 
Compare to the Bank 1 test above:
In a YouTube video on cleaning cats, in the comments someone mentioned removing the upstream oxygen sensor and carefully spraying an entire can of throttle body cleaner into the hole while the engine runs. I then cleared the codes and waited for the results. Today the readiness indicator came back and with no P420. Going to get it to the garage for inspection ASAP.
Comparing the two cat tests, you’ll notice that the Min value changed! I still want to know what these numbers mean, and the longer I’m stuck fixing this car, the more I want to see the GM Service Manual. But in either case, the new Test Value is in the thick part of the range.
If I had to do it again, I would have let the car run about 5 minutes before spraying. There are YouTube videos of people actually removing the cat and soaking it in a solvent for a few hours, but with minimal results, It occurred to me that the difference here is the cat is HOT…which makes the attached gunk that much more pliable. (Removing the cat wouldn’t have worked easily anyway, because while some disconnect to a milk jug-sized unit, this cat is welded to a long pipe.) Thankfully even my amateur efforts worked.