I would lean toward more heat to loosen the tool. Something like a rosebud heating the tool from the back side till it is red hot.

Back when I worked on oil field equipment we would get some things that had been rusted together intentionally. Heat and pressure is usually what got them to separate. Some of the old timers would use battery acid (sulphuric ) as a form of loc-tite. It would rust threads and parts together preventing slippage. I still get mad when I think about, what a pain in the rear.

Charlie