Sorry for the delay mate, here are the photos of the keg parts I disassembled with a grinder loaded with a thin cut off disc.
There may be a couple of different keg types in the pics, and they may be different to yours.
It may as well be cut it off neatly with a grinder and a thin cut off disc, then get the fitting you want welded on.
If you get it undone, you'll be left with an oddball internal thread that you wont be able to use, and will need to get the fitting welded on anyway.
I recently modified 5 kegs...
I'm a stainless tig welder in the wine industry and do stuff like this fairly often.
I would run at about 50amps with a 1.6mm electrode, pure argon.
The 1/2" socket will probably be thicker steel than the stainless vessel, so start the arc in the thicker material of the socket and feather it...