stabilised O2 sensor signal means a dead sensor, a working sensor switches between 0.1 and 0.8V constantly. and regarding the post cat sensor, its only job is to check the working of the cat converter, the ecu compares both O2 signal and if both are similar then it decides that the cat is dead and sets its error code, on single bank engine its P0420. It does not affect mixture - only the front O2 sensor has affect on it.
You can get an adapter for a spark plug and screw the 2nd O2 with it in the exhaust to mask it a bit - it will then work really slow like it should (no guarantee though)
Before any such drastic step it would be wiser to first repair it to factory spec, I can bet you it would be corrected. In my experience, ready switching O2 sensor and an insight into LTFT and STFT shows you problems more precisely. in MAF cars I found that stupid LTFT readings usually meant that the MAF was dead. In some odd cases I have also found it to be caused by a bad FPR or bad injectors too.