A magnetic field produced by electricity is directly proportional to current and the force it produces is proportional to the magnetic field strength. If that was not the case then moving coil ammeters would not have linear scales and loudspeakers would have terrible distortion. Both DC motors and steppers produce torque directly proportional to current.
Not sure what you mean by "torque would vary as the square root of the force between the rotor and the stator". The torque is simply the net tangential force between the rotor and the stator multiplied by the effective radius that it acts at. It depends on the rotor position in an approximately sine wave. I.e. there is no torque until you displace the rotor from its resting position. That is why sine and cosine currents are used for microstepping.
Not sure what you mean by "torque would vary as the square root of the force between the rotor and the stator". The torque is simply the net tangential force between the rotor and the stator multiplied by the effective radius that it acts at. It depends on the rotor position in an approximately sine wave. I.e. there is no torque until you displace the rotor from its resting position. That is why sine and cosine currents are used for microstepping.