Big news! At least for the present, the cat problem is solved…and for roughly $20!
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.
If a can of spray cleaner can make the P420 code disappear, then something is happening that is not understood.