I agree with Hillary that the MRI may be able to provide very useful information. However, I also agree with Hope that the MRI will require general anesthesia and there are risks associated with that, especially given Princess' age.
I can only speak from my own experience, and I have absolutely no idea whether Princess' situation is even in the same universe as Gwyn's. After two separate unidentified neurological events (which we much later determined to be strokes), we brought Gwyn in for MRIs. Because it wasn't an emergent situation, it took 10 days to get the appointment for the MRI machine. By then, the clots that had caused the damage were gone, so both MRIs were completely useless to us.
I'll also say that I'm a big fan of steroids. Yes, they can raise the blood sugar level, but that depends on the steroids and the cat. Some cats simply don't react very much to some types of steroids. Can you check with your vet about the type of steroid they're thinking of using, and how it reacts with the BG levels in most cats. And, while I can't remember what insulin you're on or whether you're home-testing, nor whether Princess is susceptible for ketones (nor whether you're testing for ketones), I would weigh those possible risks against the possibility of improving Princess' quality of life by increasing her chances of walking again.
If you do decide on steroids, and later decide on an MRI, please double-check with the radiologist when you make the appointment; the anti-inflammatory properties of steroids may help mask the problem that they're looking for with the MRI, and they may request that Princess be off steroids for several days before the MRI. (This did actually happen with Gwyn: I specifically asked them if they needed Gwyn off steroids for the second MRI, they said no, I brought her in, they asked about current drugs, I said steroids, and they refused to give her the MRI. We had to re-schedule for another ten days out, which was 12 hours before I was scheduled for abdominal surgery. And, while there are a few MRI centers dedicated to pets, most pet MRIs are done after-hours in human labs. So we had to get Gwyn first in line for the machines that night, wait for her to recover from the anesthesia, and take her home AMA at 3am so that I could get her settled back home and me to the hospital. Of course, I got home from the surgery a few hours later, we were both feeling pretty awful, and all Gwyn wanted to do was curl up on my stomach for comfort ... )
You're both in my thoughts and prayers --
Jean and her Gwyn
ETA: If your vet is suggesting prednisone, you might see whether you can get prednisolone instead. It's a slightly different form of the same base drug, and is often more effective so you can get by with a smaller dose.