can you get to the thread from the other side?,a pair of mole grips might be able to remove it now the head has come off the bolt.
This is what I would do, do not take this as professional advice.
Soak in penetration fluid for a while.
Cut in a groove with a dremel
Blow torch it for a while
Chisel in the groove and wack it off center to turn
That to me looks like a captive nut like these... I've seen enough of these in my time with VW restoration! I think you'll find it was pushed into the hole from below, then spot welded in.I was thinking that, however the only way in is to break whatever weld is there and try to go in from the end.....
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Other alternative i have thought of - try to fracture the weld where on the bolt "holder" and get that out... nothing obvious as to how it is held on - suspect it may be spot welded... might cause less damage though....
Damaged screw extractor set & bolt remover is made from H.S.S 4341#, the hardness of this screw extractor set is 62-63HRC, which is stronger than ordinary steel,