Good afternoon, I hope I can help you.
It sounds like the next step will be a replacement ecu.
The garage are obviously throwing things at the car in the hope that it will cure it.
There isn't much left except the wiring harness and ecu.
If you could get the fault codes that are present I might be able to help further.
Any air leak in the exhaust system before the o2 sensor can cause it to give false readings and doesn't always mean the sensor is faulty.