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 story that you present are non-specific and could represent a number of different pathologies. Just given the symptoms that you describe (without knowing the further testing) I would suspect a brain tumor, multiple scelrosis, a stroke, or even possible ALS. However, the testing that you decribe make these possibilities less likely. I would have expected the MRI of the brain to be abnormal if the your symptoms were due to a brain tumor or MS. I would also have expected your EMG to be abnormal if your symptoms were due to ALS (which presents with fasiculations (twitching), weakness and atrophy). I agree with your doctors, that you need an MRI of your spine to evaluate for demyelination, tumor, vascular malformation, etc. If this is also negative, you may consider a lumbar puncture (for inflammation, IgG index, oligoclonal bands), visual evoked potentials (to look for past evidence of optic neuritis) and a somatosensory evoked potential (to look for functional deficits in your spinal cord). If nothing is found then I would suggest that you should go to a major academic center to have your tests repeated and get a fresh perspective on your case.
I hope this has been helpful.