Mechanical dysfunctions of the back can cause the muscle spasms you're having and pain radiating into the buttocks, thighs and down the leg into the foot, but your right nerve compression is a more common cause. The other thing is MRI’s can miss things. What you wrote about the exterior digitorum longus muscle is interesting because the nerve root at L4-5 the L5 nerve root supplies the exterior digitorum longus muscle, and part of the front edge of the knee is in the L5 dermatome.
Regardless of the cause, you would probably benefit from seeing a good Physical Therapist. Strengthening of the abdominal and back muscles will help relieve your symptoms.
Take Care
Your feet are your foundation! Alot of times people that have feet problem develop back problems. Any chiropractic or podiatrist will tell you that as well. So yes, if you have been having foot problems, your back pain can be caused from the foot/feet.