For really hard cases do the following:
I finally got the retainer out. It has, as predicted a left oriented thread.
Since all the other solutions would not work with my really hard stucked hub without leaving serious marks , I decided to go for another solution.
Here's the retainer and hub-friendly solution I suggest:
Take an old sprocket and weld some flat steel on it, get some 8mm hardened bolts out of your local screw store and bore 2 fitting holes into another flat steel with about 1m length. The product, fully mounted, looks like that:
The only thing is, that your bench vise should be mounted firmly. Mine nearly broke it's mounting bolts into half while pulling on the lever.
Here's just a view from above, and let me tell you, that I still had to pull reaaaally strong. (Man, who had fastened the retainer that firm??)
Regards