The best penetrating oil I have ever run across for something like this is 1/2 acetone and 1/2 ATF (automatic transmission fluid). takes half the torque of Kroil, or any other brand I've found. Those two ingredients will not go into solution, so shake it hard in a pump spray.
I've had one that was so seized with the 1/4" key that just had to be cut off with cutting torch. I try to save all I can, and it broke my heart, but why spend 5 hours trying to save it. Came off a unit that someone never serviced since new, and left it out in the elements completely and the zerk was gone.