The LP is only effective for 85% of people who are eventually diagnosed with MS. I have had 2 LPs with 20 years separation and I have never had a positive LP. The LP is just one "cog" in the process.
It is not uncommon for lesions to be overlooked. It takes a trained, experienced eye and it also takes luck in timing. I certainly realize the frustrations you have been enduring.
I will keep you in my prayers. Please know that many have been in the spot you are now. You are not alone.