The rotor should come off with a little wiggling with the park brake disengaged. I've had no need to slacken the park brake adjuster and definitely no need to detach the cable. As it is I don't think you can detach the cable without removing the rotor. You'd have to drive a long way with the park brake engaged to wear a groove into the drum.

If I didn't have the rotor off to see the park brake adjuster, I'd never have figured out how to turn it using a long screwdriver through a lug bolt hole. Even with the manual showing how to do this.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD