It sounds like you and Mr C have trapped yourselves a good MS nurse. They are truly worth their weight in Lindt dark chocolate. Hang onto yours. (By which I don’t mean lock him in the cellar!)
(I always like people who agree with me!) Since your MS nurse also thinks IV steroids are pointless, in your situation, I’d say no. The reason is that a) supposing it is progressive MS, the steroids won’t help because strictly speaking, it’s not an inflammatory type; b) supposing it is RRMS, the steroids won’t work because the relapse was too long ago - ie the inflammation stage has passed.
Steroids are evil bad nasty drugs. This doesn’t mean I don’t take them when I need to. In fact, I took a 5 day course of high dose steroids last week because even though I’m officially SPMS, I seem to have had a relapse (no one ever said MS doesn’t have a sense of humour!). They aren’t nice drugs to take, they make you a bit hyper when you’re on them, and give you a come down afterwards. They leave you feeling bruised, like you’ve been brawling with a prize fighter. There are other side effects: https://www.mstrust.org.uk/a-z/steroids-methylprednisolone
I suspect that the next MRI is going to be more useful than anything.
By the way, when the neurologist said if vertigo from 15 years ago is included in the diagnosis, then the diagnosis would be classed as RR. Why in that case, doesn’t s/he err on the side of including it? And prescribe some DMDs. If that is possible, it makes sense to me that it wouldn’t be a bad thing. And DMDs could ward off relapses, if it is indeed RR.
Obviously, as always, this is my non medical opinion. I have no scientific or medical basis for my thoughts, just a bit of experience and a touch of common sense.
All the best.