You said "I mean, I've always thought that anxiety/depression is a complication of mTBI in extreme cases, and something that only rarely manifests itself in PCS."

Anxiety is likely the worst and most common symptom of PCS. It appears to have a firm grab on you. Many who suffer a concussion already have a propensity for anxiety. For these, a very minor concussion with mild symptoms can often be almost overwhelming.

Regarding these taps and concussions or other. It does not make any difference. If these taps, head turns, etc. cause symptoms, then they should be avoided. When they can not be avoided, one needs to learn how to move on and just deal with the symptoms. To paraphrase the saying, Hits happen. Same letters, just a different order.

When these friendly touches happen, I would start with a firm statement. "Do not touch my head." There is no justification for such a behavior, even as a friendly gesture. For me, it is not the touch that is the problem. It is the head jerk afterwards that can effect my head or my neck.

And please try to stop trying to label everything. It is a negative effort. Concussions and mTBI's are just terms to define clusters of symptoms after an impact to the head. It is only the symptoms that matter, especially at the lower end of the impact scale.