You are to the point that you need to soak that SOB in Kroil or PB Blaster overnight and then just commit to getting it out. Keep spraying either one while you are working on it.
You know that an adjustable wrench has it's place, but putting ugga dugga's into something is not it's place. I would strongly suggest you drill out the center of the sensor and see if that releases some of the binding. Then you put a 6 point deep well on it and with either an impact or a 24" breaker bar, alternate between tightening and loosening motions, work that damn thing out of there. You have to be more stubborn that then sensor and I am willing to bet you can be pretty damn stubborn.
I know you are afraid of damaging your collector pipe, don't be. Your choices are a stuck sensor, a potentially damaged bung that is fixable at an exhaust shop and a sensor that is out and then some threads to clean up and then put a new sensor in.