Hi there
Real PITA these rear wipers! I've had to deal with several of them.
First off, if the pulling by hand method doesn't work, have you tried a dedicated 'wiper puller' tool?
If so, and this still wont shift it, then I would recommend cutting the side of the wiper arm where it goes over the motor spindle. You can either use a hacksaw or a small angle grinder. The outer plastic will give first allowing you to take the arm off and out of your way - this will leave you with a ring of alloy on the steel threaded section of the wiper spindle. Now turn your cutting tool to this and it will give up its corroded bond. Just make sure you cover your tailgate and bumper with a sheet to protect you glass and paintwork from swarf/grindings.
Obviously if the wiper is going back on you will need a new arm.
Hope this helps!