Short crank arms will help with limited range of motion. Typical road cranks are only available down to about 165mm, which may not be short enough to make a significant difference. TA offers arms down to 150mm, and BMX cranks can go even shorter, 110mm, but may need some modification to take multiple chainrings if you require that.

Can you get a referral to a sports medicine physical therapist? That would help in determining how to modify your bike to meet your needs and also improve your stability and range of motion.