The rear O2s are NOT just for monitoring the CATs.
Do a search on this forum as I have posted GM info as to what rear O2s do

They do play a part in what the PCM decides is the average part throttle injector pulse width for a drive cycle

The rears are more trusted on start up since the fronts have more of a temp change and since AIR is injecting air the rear O2s have a more stable exhaust makeup and temp range.

Rear O2s due to their wire length require more current load thus the rears are not the exact design as fronts thus should not be swapped front/rear

Rears should I think function longer as temps and exhaust is better but I would not want old fronts in the rears

At the least replace the fronts, review this forum as I list the new GM designed O2s and their part number

E10 or E85 due to the ethanol will change the fuel trims as it is supposed to be richer then regular gas but it takes more volume of it to produce the same HP as normal gas

Yes as of 2 years ago EPA was requiring using this junk in all grades and of this year most gas station are even using higher percentage of ethanol and less performance and MPG is seen.

Also our cars were never designed for it and can effect O2s and even cause internal rust in fuel system to break off and clog pumps, filters and injectors.

Reports are ethanol, esp E85 drops MPG from 10 to 20 %