First of all, keep in mind that I am unable to diagnose you because I am unable to examine you, this forum is for educational purposes.
The symptoms and history you describe is not specific any any single disease, but is concerning for number of different possible disease processes.
The finger flick test with thumb movement is called Hoffman's sign and indicates hyperreflexia in the area of the cervical spine. The symptoms that you describe of hyperreflexia, pain, numbness, weakness and atrophy are consistent with a spinal process. Given your initial negative MRI of the lumbar spine, I would recommend an MRI of your Cervical spine. Disc disease, inflammatory disorders, tumors etc, in the cervical region and affect the areas of the leg and cause the symptoms you describe. Another possibility would be an upper motor neuron (in the brain) process (causes spasticity). To evaluate for this you should have an MRI of your brain. You should also make sure your MRIs are done with contrast. Finally, you should have a lumbar puncture to look for inflammation(such as multiple sclerosis), lyme and cancer in the spinal fluid.
I hope this has been helpful.
I didn't tell you maybe the most important thing, I just finished a year of enbrel, and I am scheduled for a brain and cervical MRI this week.
Sounds like you need to have imaging done of your brain as well as your cervical / thoracic spine to make sure there's nothing going on there. Spastic changes usually take place higher than where "sciatica" does, and that means anywhere from your thoracic spine and up, including your brain. You seem to be normal, otherwise, but there may be a small problem somewhere along the course that's irritating your "foot nerves" tract.
Go to a movement disorders specialist (special neurologist)