Vincent,
A shocking situation and you have all of our support here. If nothing else, keep venting your anger, we will listen.
All I can suggest is to further explore other doctors / hospitals until you strike someone that will progress with something. I know its far easier for me to say it than you to physically do it but you gotto keep trying, for your Mom's sake. It is really important that she gets some quality of life back and also for you too.
perhaps, you share the workload with your sisters and put a standard set of documents together, which you pretty well have already including copies of the latest MRI's and radiologist reports and a standard letter outlining the history and current situation and where you'd klike your Mom to be - eg, little or no vertigo etc.
have these ready on your computer - it would be great to scan these reports and images into your computer so you have an "e" file of it all. Have your standard letter ready to just change the name and address.
Between yourself and your sisters, get a list of hospitals and names and ring them all up. Hopefully you'll find one that will take a further interest.
I know its hard yakka but don't give up.
Laz