Hi and welcome,
Your saying you've been experiencing intense pain that travels halfway down your back (spine) when you bend your neck forward as little as an inch or two since you were a child of 12 or 13, and despite numerous tests and different dr consults you still have no idea what could be causing this.....and now your wondering if it might be Lhermitties and a sign of MS.
Technically the Lhermitte's phenomenon generally occurs with pathologies involving the cervical spinal cord but it is not specific in origin, so whilst it is associated with MS, even if what you've been experiencing since childhood was determined to be Lhermitte's, a spinal cord lesion (ie MS) would only be one of the possible causes.
Honestly, neurological conditions like MS would be one of your least likely causes specifically due to it's paediatric beginning, lack of developing any additional MS related symptoms over an 8 year time frame and what i can only assume was the absence of suggestive-consistent objective test evidence of spinal cord lesion(s) at the time....
Thinking of test evidence, because you were a child at the time you may not be fully informed about the specific details of your test results so i'd advice you to get copies of all those reports and consider seeing another orthopaedic specialist for reassessment now that your an adult.
Keep in mind that 8 years is a long enough time frame for things like developmental spinal issues eg mild scoliosis, an over curvature of your cervical spine etc etc to be more clearly evident now your spine is fully developed, which btw is actually what happened with my daughter, who around the same age as you did also first started experiencing back problems but she wasn't diagnosed with a double curvature of her spine (mild scoliosis) until she was about your age, her childhood tests showed nothing specifically abnormal but her adult tests clearly showed her spine didn't develop straight like it's suppose to....just a thought for you to consider that is unrelated to neurological explanations like MS.
Hope that helps......JJ