Removing Seized Anode Rod From Water Heater
You have certainly given it the effort to remove that stubborn anode rod. More intensive effort could very well damage the unit.

With a water heater that sounds like it is over 20 years old it might make sense to simply replace it. You have gone to all the trouble to remove it from its hiding place now would be a good time to place a new one with higher efficiency ratings in place instead of the old one. 

Another thing to consider is that if the anode rod is that corroded there may well be other parts that are similarly corroded. There could even be corrosion pitting of the tank itself that may soon penetrate the tank and create a leak.