First, sorry you are dealing with this.
You need to rule out everything else first, injuries, arthritis, etc. Then you can do a quick and cheap swap test and see if she actually could have DM. I think OFA has them. Personally I wouldn't do the MRI, but that's just me.
I can tell you that DM doesn't usually happen quickly. It isn't normal for the symptoms to suddenly appear and usually the first thing owners notice is the sound of the dogs nails dragging or scuffing.
If it is DM, swimming is awesome and exercise is a must, diet becomes very important and proper weight must be maintained.
Stem cell treatments, IMO need a lot more research. Some have reported success, some haven't.
My vet told me ultimately it is a disease that comes down to how much you can take, not the dog. Some owners hang on to the bitter end with the dog completely immobile and in diapers. I couldn't and neither could my dog.
Although some dogs hang in there for a lengthy period of time, average time from onset of symptoms is 8-18 months.