[ I would assume that it takes time to resolve the constellation of chronic pain and disabilities that accompany TOS even after the decompression surgery has been successful. ]

Correct!! muscle memory and possible long term posture issues , will need to be addressed.. IMO...

Was surgery this year or last year?

It can take a lot of time to fully recover so much internal tissue was moved around and stretched during surgery most likely...
Gentle ROM /stretching , posture alignment, overall movement are all good things to do..

I used to get the ice pick pain if I allowed my shoulders roll forward too much..

check out the sticky threads/saved info and use the site search for specific topics