[JimsCAL]

Pretty sure its threaded onto a fitting on the end of the cable. Try soaking it in PB Blaster or something similar for several days. Vicegrips on the cable fitting and a wrench on the handle "should" break it free. Good luck.

 

[JimsCAL]

It makes sense that it'd be a threaded fitting and there is indeed a locknut which is easily undone, but when I go to 'unscrew' the handle I'm only twisting the internal cable.

As I suggested in my first post, try holding the cable with Vicegrips. That should allow you to apply enough torque to the handle to unscrew it.

 

[JimsCAL]

Good news! Glad my suggestion worked for you.