From the pix of the remains of the freewheel it looks like the pawl pocket is your best point of attack.
Get a file and convert the pocket into a flat. Should be two pockets, flatting both will allow you to
clamp the flats into a vise and use the rim of the wheel as a wrench. Thread is R hand, just make
sure you are unscrewing it the right way. Also make sure when you file that you don't file a taper
in the flat and the flats should be parallel to each other or it will cam out of the vise. And don't file through to the threads. Steve