Hello, Wallace - I don't know this fitting like you do by now, but I did learn from other soft German fasteners years ago to, yes, soak with solvents - - and to shock the parts with hammer & broad-faced punch at various times during all that soaking.
And sometimes German Judo helps, sometimes not; apply your turning force clockwise, so that you're not fighting that corrosion buildup on the inside of the threaded section. Ideally, it will give you just that little bit of 'wiggle room' needed to begin Phase Two of your current character-building exercise. This used to work well with fasteners that had corroded in their original factory-assembled positions and would just twist off about 9 times out of 10 if approached counter-clockwise.
And then if it still won't listen to reason, you might saw it off back to the flats or so, then use a large dullish cold steel chisel applied to that "soft" fitting parallel to the bore in order to collapse it, pulling much of the threaded portion away from its mate in the head. You want to break it inward rather than cut it, so that the break carries into and through the threaded portion in the head. That ought to show it who's boss.
OK, wise heads of the forum: will the head itself stand up to such treatment of this fitting?
Denny