--given the onset and the pattern, this sounds like it's compressive to me.

Given the "clear" lower (lumbar) spine MRI, and the fact that this started in the upper back, I definitely think you need MRI of the cervical and thoracic spine. Compression of nerve roots tends to produce symptoms in specific places (known as the root's dermatome), but spinal compression itself can produce symptoms ANYWHERE from that level downwards, depending on what's being pressed on (and a competent doctor should know that--why your first doc did not set you up for a cervical MRI at least is beyond me).