If your rear view mirror has the red and green lights on it I do not belive you can use an ME unit from another vehicle as it is married to the VIN.

I would encourage more tests regarding the O2 sensor circuit. It is possible that the ME is at fault; however, with the read codes, monitor actual values for a short time, replace a component then clear codes approach something may have been overlooked. You have every right to request what tests were performed in order to lead them to a conclusion that the ECU is the problem.

A 2300.00 oops that wasn't it now what situation is uncomfortable to both parties!