PWM will affect torque, but not necessarily linearly. Current controls torque, PWM controls
effective voltage (linearly or not depending on decay mode), which affects current (depending
on motor speed).
For a stalled motor you need to be careful not to wind it up too far and fry the motor, there's
very little cooling a stationary motor. You will need gear reduction, enough to give reasonable
torque, but not so much its friction prevents back-driving of the motor.