Given the design of that motor axle a torque are made from mild steel will not works so you can forget that avenue. The outer slotted washer shown in the bottom two pictures is suppose to take the place of a torque arm. Given what I see in the pictures I would likely build something like the drawing and use the two existing screws to secure it I would not involve the axle at all.
You have a definite chain problem and a definite rear derailleur problem just from the pictures.
