While I can offer no help to your problem, I feel your pain. I spent 4 hours trying to remove the nut on the rudder shaft. I even put a pipe on the wrench handle and bent the pipe. Nut is still on.....
Have you tried heat? (If you can.) I had a plumbing fitting in the basement that was similarly stubborn. Wouldn't budge for love nor money. Then I thought "Hmmm... I wonder...?" Put a bit of heat to the "outside" fitting with a plain ol' Bernz-O-Matic torch and voila!, off came the fitting with ease.