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.
Your case sounds very complex and there is much data that I do not have. As for your question, could this be mecury, the answer is yes. Mecury poisoning generally starts out with sensory symptoms (Most frequent is numbness/tingling in the fingers/toes and around the mouth) then gait problems (stumbling) develops and varied other symptoms including weakness, inability to concentrate, fatigue, etc. In the worse cases people have developed tremors, jerks, seizures and some with very high levels/acute exposure have died. Mercury can cause changes in the white matter and deep nuclei (including the globus pallitus, thalamus, putamen, etc), whcih may explain the findings you suggested. There are chelation therapies available (penicillamine) but chelation has not been shown to improve the rate of recovery in all cases.
As far as MS is concerned, it sounds like you have had adequate MRI testing, but have you had a lumbar puncture or visual evoked potentials? These would round out your case if not yet completed.
L'hermitte's sensation (the feeling of shocks, stars, etc. in the neck when the cervical spine is flexed is a non-specific finding. It can happen any time there is damage to the upper spinal cord, including MS, spinal cord tumor, severe cervical disc disease with spinal cord compression and with toxic damage.
Test for the leukodystrophies is also appropriate, as there is more found all the time, and we are seeing less symptomatic patients presenting at later ages.
I hope this has been helpful.
I am sorry to hear of all of the confusion. I can offer advice on the Lhermitte's Sign. LS, is caused by disruption within the cervical spine and is aggravated. Most commonly hallmark in Multiple Sclerosis, though it is not specific to MS. This can be caused by anything from an annular tearing or a srynx to lesions related to a conditions such as MS or Lyme disease.
Good Luck!
JCmcc.