Not that it helps much, but there are “california approved” catalytic converters, still over a thousand dollars for the ones that come right off the block though.
The rear O2 sensor will bounce a little as far as I know, but should stay consistently rich and not follow the usual oscillation you would see on an upstream narrowband o2 sensor.
As I was discussing in another thread regarding a Mazda I do believe that at least some cars will use a rear o2 sensor will “calibrate” a front o2 sensor, but I haven’t found much information regarding what cars use this strategy. Are you seeing a big discrepancy between bank 1 and bank 2 rear o2 sensors?