Hi and welcome fellow Ozzie,
Technically MS is still a clinical diagnosis, symptom type and pattern are very important to diagnosis but active enhancing lesions in both your brain and spinal cord definitely meets the internationally used MS Mcdonald criteria, you still would of met criteria if you'd only had both old and enhanced lesions show up on just your brain MRI, add the spinal cord evidence and you already had the MRI evidence of at least 2+ attacks 4 years ago.
I don't understand why your neuro needed further evidence of demyelinating lesions before diagnosing you with MS, nor do i understand why your neuro is not calling this a new relapse because of your MRI when your additional new clinical symptoms are clearly indicating you've relapsed!
Previously experienced remitted symptoms that return 'and or' worsen 'and or' you experience any new symptoms etc equals a relapse....in your situation your experiencing symptoms in the opposite side your body that you've not experienced before and would typically be from a new demyelinating attack, a new relapse and not a pseudo relapse which is the temporary return of pre-existing issues that last until you cool down, rest etc, so regardless of if your MRI showed new lesions, enhancing lesions or not your clinical signs are enough to call it a relapse...
The type of symptoms you've mentioned could be due to another spinal cord lesion which for many technical reasons can be a lot harder to find but to give you an example some of our community have had no spinal cord lesions show up on a 1.5T but on the 3T it showed they actually had multiple spinal cord lesions.
I would 'highly' recommend you seriously consider seeking a second opinion with an MS specialising neurologist! Wait and see is not the usual way anyone treats an MS relapse that is severe enough to effect mobility, IVSM steroids are not recommended for minor symptoms but for mobility they typically are......waiting to dx you, holding off on steriods because of the MRI etc makes me wonder if he's even started you on any disease modifying drugs to slow down and or alter your MS yet, regardless of if you have or haven't i honestly think you need to seriously consider getting the second opinion with an MS specialising neuro who will at least be more up to date on everything MS related!
Hope that helps......JJ