Living in the salt belt.
I found it impossible to take the down stream O2 sensor out.
Had to take it to a garage and pay $50 dollars to get that sucker out.
Acetylene torch and plumbers wrench. It came out.
But, there were no threads.
The salt had made it so bad that there were no threads left on the sensor
Got an O2 tap to chase threads to get new one in.

Your description is exactly what I experienced. Very thorough. I'm sure it will be appreciated.