Hi Thowell, this is Akash from India - I suffer from chronic neck pain and various issues due to an accident. Over the years i have had to do my own reading.
"Dr. literally walked in and without even looking at dd12 decided it wasn't an Orthopedic issue!"
This makes me wary. At the very least he should have examined the shoulder neck girdle in a variety of positions.
She underwent, x-rays, catscan, MRI, and blood work. She was seen by, Ortho, Neurology, and Physical Therapy. Noone seemed to have any idea what was going on. The only thing that was ever identified is that she had a lack of curvature in her cervical spine due to muscle spasms.
My question is how good were the PTs. Were you happy with them?

Chronic neck pain can be due to a variety of reasons. The complex ones are due to stuff like Chiari and disc herniations or disc bulges. These require MRIs- have they been ruled out?

If its one sided and spasms - is it on your daughters dominant arm side? In either case, check her shoulders. If they are flat or the clavicle slopes up, upper traps are strong, and perhaps overactive. If they slope down, the clavicles, that is, chances are her Upper Traps are weak.

You see the upper traps (neck extensors) pull on the neck and to compensate, the neck flexors (SCM, scalenes) go overactive causing nerve compression etc. A common cause of Thoracic Outlet Syndrome. In short, please have her PT check if she has depressed shoulders, if its weak upper traps a very simple answer could be strengthening the Upper Traps so they do their job and no longer pull on the sensitive neck in a stretched position. On the other hand, if her shoulder is higher than normal, its the reverse, you need to strengthen the lower traps. Again, in the US, your chiro or any PT may be able to help you with this. Since her issues started without any specific trauma, that is (to my amateur mind) a positive thing and sometimes, simple things can make a bunch of difference. Another thing is whatever action she does with the painful hand, have it supported (say an armrest on a chair, the muscles dont pull on the neck).

Next, when she sits or stands, have her head back on her shoulders. Forward head posture and using the arm worsens things.
When I got my neck injury, the entire kinetic chain starts freaking out. Pain in back, calves, feet etc all piles on.
Dizziness, etc - if the more complex issues are ruled out, can often be due to spasming neck muscles pulling on the spine and irritating the structures.

Best of luck and let us know how it goes.