this maybe really stupid but can't you simulate the 02 sensor just to see what effect it has on the gas readings as i thought this was the purpose of sensor simulators. I would think the 02 sensor should react to lean/rich mixtures even with a poor functioning MAF sensor which don't normally log a code in my experience but seem to log 02 sensor code.

LTFT adaptions are normally a guide to faulty MAF sensors but this doesn't seem the case with yours as the 02 sensor isn't switching fom 0.3v so must be a fault with that. Have you tried squirting carb cleaner up air intake to see if 02 sensor reacts to rich mixture.

very basic but just a thought.