In the past, I have helped a friend of mine remove a tight headshell from a Garrard TT which was in storage for about 30 odd years. What we did was we carefully applied a bit of heat (using the side of a soldering iron) on the metallic screw join and gently tapped the warm joint (all around it) with the head of a small screw driver (very gentle, like tapping a little baby), after this we managed to unscrew the headshell and dislodge it. However, this is a risky proposition so please attempt it only after some trial runs on a piece of aluminium pipe or likewise.