First, in my experience, the metallic taste associated with a concussion happens immediately. I have never experienced a delayed metallic taste. And, that instant metallic taste is momentary and goes way within a few seconds.

I don't think anybody other than your CBT therapist and you yourself can convince you that leaning your head against the wall will not cause a concussion.

It takes 40 G's or more to cause a concussion. Leaning your head against the wall would not even reach sub-concussive impact levels.

Unless you are taking an SSRI or such, the sweating has no connection to PCS. SSRI's can cause sweating. Sweat has salt. salt stings the eyes.

Your problem is anxiety and refusing to accept logical truth. That backpack, unless it had a heavy solid object in it (bricks or the like) did not cause a concussion.

Hormone imbalances can sometimes cause facial sweating and dandruff.

When you shower, put the anti-dandruff shampoo in your hair first. Then do your showering and rinse the shampoo out last. The time the anti-dandruff medication is in your hair will help it work.