Time for some vise grips maybe? It's not stock so who cares really if you wreck it. I'm guessing they made a hole under it just the right size or smaller than the shifter needs and just hammered it on.
Yeah, finally I got some advise today from quapman how to remove the whole center shaft. My thoughts are now:
When removing the small cross pin near the bottom of the shaft it should be possible to pull out the center shaft with my unremovable metal trigger button. Then I can take a closer look, what is wrong with it and I can even remove the shifter plate and woodgrain knob, which is still on and disturbs me.
And yes, then I will wreck it and wait for the Auto-Grip.
Will take a short look right now.
I'm writing this here, cause maybe one day someone needs the information too.
CU
walkAbout