bas,

We often discuss Upper Cervical Chiro benefits. These are not run of the mill chiros. They are specially trained and use gentle techniques. Your ST is over-selling the scare about chiros. In the US, MD doctors' mistakes KILL over 180,000 patients per year. The risk for chiros is very small. The low premiums of their malpractice insurance is a good example.

But, I always recommend caution about letting a chiro twist and pop your neck, especially after a concussion.

I would be more concerned about an aggressive massage therapist working on my neck.

Osteopaths also have some techniques that use the patient's own muscle force to treat the neck.

Your therapist sounds like a classically trained sports trainer / kinesiologist who likely took a weekend course on treating C-1.

You commented "Do you have this weird feeling of weakness/malaise/anxiety?" This is a very common symptom of PCS. Tell us more about your struggles and what you are doing daily. Brain fatigue is a common cause of malaise.