... and here I suspected the dandruff against dark background reminded him of his experience floating through time/space
It's dandruff. Look at the scene where he's on the phone to Mr Todd. It's clearly there on his shoulder.
Definitely dandruff. I had the thought, though, that against the black suit, it may reminded Dougie of the stars he fell through, and then gazed at on top of the little cell as it floated in space in part 3. I suspect he was trying to "touch the stars", and certainly not to give a heart-opening massage.
... and here I suspected the dandruff against dark background reminded him of his experience floating through time/space
That's what I thought, too.
I agree with the acupressure theory - Coop's reliance on the Tibetan Method would suggest that he is possibly also knowledgeable about Eastern medicine.
Coop, deep inside Dougie, did know that Anthony with the bad dandruff was guilty - he had cracked the case wide open for Bushnell. There seemed to even be a flash of skepticism on Dougie's face when Anthony invited him to coffee.
I believe that though Dougie was initially compelled by the dandruff, it led him to subconsciously apply Coop's acupressure skill to places on Anthony's back that are associated with the large intestine meridian, a line of acupressure locations that are associated with internal organ cleansing and release of mental anguish and guilt, triggering Anthony's confession.
So many things with Cooper seem to work by coincidental happenings. Remember back when he was leaving with Jade, how they went over a speed ramp and he dropped the Great Northern key he was staring at? Had that not happened, he'd have had his head blown off.
Fire walk with