NP is variously called rare as well as common. Its commonly missed because itch can be transient even if later it becomes constant or more severe. Usually its a dermatologist who makes the diagnosis since it is the skin discolouration that prompts care-seeking. But you are right - injury can often precede such a condition. Even now it is unclear exactly where the nerve damage is likely to be. It can be anywhere along its pathway as it exits the spine and provides innervation to that patch of skin mid-line to the shoulder blade. The nerve roots naturally glide in and out between adjacent tissues with natural movement, and the nerve also does this between the layers of connective tissue or muscles it passes before it reaches the nerve-endings in the skin - branching as it does. The posterior ramus of the nerve root in the thoracic region takes a few sharp bends and can be irritated or compressed along its path. These irritating forces are exacerbated by arthritic spurs, increased thoracic curvature, tight spinal muscles in addition to local inflammation. At this stage I don't think anyone knows why this becomes an itch in this region rather than pain or numbness as it might in other peripheral neuropathies. Itch nerves are rare, but when irritated they appear to recruit adjacent pain fibres to amplify the itch sensation. There are many other causes of itch which can influence the threshold of itch - so that many treatments only work partially - exactly as you have reported.
Whether the disc problem in your neck is involved in the thoracic area itch is unclear. I say this because nerve compression in the neck usually affects nerves into the arm and shoulders- rather than the midback. Usually the midback itch is from compression of thoracic spine rami. Cervical disc irritation MIGHT compound this midback pain because nerves innervating the cervical disc itself can refer to the mid-back region, Again no explanation can be given regarding why its itch rather than other symptoms.
I haven't spoken with anyone having a diagnosis of notalgia paresthetica in many years, although I understand it is common enough in people aged 40 - 80.
Usually a dermatologist discovers and makes the diagnosis.
You're right -- we don't know much about it, other than it seems to be a kind of neuropathy. The reasons behind notalgia p. are unknown.
And with your cervical MRI report, I wouldn't consider seeing a surgeon. Surgery could do more harm than good for now. Your c. spine disease is mild, and you may get some relief from PT, then again with your Dx, you may not.
There is some success in treatment using TENS. Ask your doctor about a trial with a TENS machine. If TENS works for your pain, what a simple solution!
Nortriptyline is what's called a tricyclic antidepressant (TCA), and chemically, it is a drug similar to an antihistamine (that's how it was discovered -- looking for a better cold medicine.) TCAs are effective against certain kinds of pain. If you have limited side effects, then use it. It might help with sleep, also.
Best wishes.