Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Lily has suddenly lost the ability to support her weight on her rear legs.
Did the veterinarian at emergency give you any idea on why she is unable to support herself?
Does she seem to be able to feel her rear feet toes?
Is she able to urinate and defecate normally?
Unfortunately Pomeranians as a breed are prone to a problem with their intervertebral discs, which are the spongy cushions between the individual vertebrae in their back and neck. These spongy discs can move or rupture and place pressure upon the spinal cord which can lead to pain, and in severe cases paralysis. I suspect that this is the trouble with Lily.
Radiographs can sometimes be diagnostic but often early on in the disease process, because the discs are soft tissue not bone, everything will look normal. An MRI is the best way of diagnosing disc disease.
If the dog is painful but has no evidence of paralysis we can try strict rest, anti-inflammatories and pain medications for several weeks to allow healing. In that case an MRI isn't absolutely necessary.
If there is evidence of weakness or paralysis then surgery by a board certified veterinary neurologist, as soon as possible, is indicated. An MRI is indicated at that point as it will identify which discs are affected and thus direct the surgeon.
Surgery is no guarantee that she would regain function, but it is her best chance. Surgery and the aftercare needed can be thousands of dollars.
If you know surgery is not something that you could or would have done for Lily then an MRI isn't absolutely necessary. It would confirm the diagnosis, but not change medical therapy. They can try anti-inflammatories and see if she comes along. But once dogs lose feeling their prognosis for return to function becomes very guarded, thus the veterinarian's comment about possibly this being the end of her time with you.
So in short if surgery is not an option then I don't think an MRI is absolutely needed.
Please let me know if you have any further questions.