Since the tub is already trashed there is no need to remove the drain thingy. Go around to the other room and pull that insulation from under the tub. You should find where the drain thingy connects to the plastic drain piping. There will be a trap between the tub and the drain. Between the trap and the tub there should be a slip nut connection, merely undo that slip nut.
OR, you can cut the piping between the tub and the trap, probably with your sawzall.
BTW, if the tub makes a solid thunk when you hit it with the hammer it is cast iron. If it sounds rather tinny it is pressed steel.