Hi, welcome to the forum. It is sometimes hard to know all the facts about someone that a neurologist knows, so we cannot say anything definite about your case. I have a couple things to say about what you have told us.
The Optic neuritis doesn't NOT mean that MS is inevitable, even when it is recurring. If the ON were occurring alone (without the numbness) with a negative MRI, your chances of developing MS within the next 15 years is about 25% according to the very large ONTT (Optic Neuritis treatment Trial) which has been going on for the last 20 or so years. It's results were just published. We just had a post on it:
http://www.medhelp.org/posts/show/664930
The fact that you have other symptoms (the numbness) would affect this chance, I believe, but I do not know how much.
BTW - What were the signs of inflammation that showed in your LP? Was one of them an elevated "IgG Index?" If so, then your LP would be considered positive for the purpose of diagnosing MS.
One of your symptoms bothers me a lot. Primary joint pain or joint pain that is occurring in several joints around the body is NOT a sign of MS. It makes me want to have you evaluated by a rheumatologist before you settle on MS as the most likely diagnosis. I am assuming that the neurologist sent a whole wartload of blood tests looking for the mimics of MS. Many of these are inflammatory diseases which also cause arthritis.
So, it would not be unreasonable to request a Rheumatology evaluation to look for one of these mimics before starting an MS therapy. And yes, I do believe that starting therapy early, even before the diagnosis is a slam dunk, is appropriate.
I do believe that it is possible to have MS with a negative MRI and also a negative LP, but it would be very uncommon. That is why the search for another cause of the optic neuritis and the joint pain makes sense to me. I don't mean to second-guess your neuro. But, I hear hesitation in your voice and feel you should be as confident as any of us can be, especially with a negative MRI.
I wrote a blurb on how you can have MS and still have a negative MRI for our Health Pages (upper R corner). That might help with all this. One question is what strength machine the MRI was done on. Especially on the spine the MRI strength can make a big difference.
Maybe you can give us more information about why your neurologist says that full-blown MS is inevitable for you.
Quix