I finally located the origin of 100 dpm per 100 cm squared in NRC Inspection and Enforcement Circular 81-07.
"For smears of a 100cm2 area (a de facto industry standard), the corresponding detection capability with a thin window detector and a fixed sample geometry is on the order of 1000 dpm (i.e. , 1000 dpm/100 cm2). Therefore, taking into consideration the practicality of conducting surface contamination surveys; contamination control limits should not be set below 5000 dpm/100 cm2 total and 1000 dpm/ 100 cm2 removable. The ability to detect minute, discrete particle contamination depends on the activity level, background, instrument time constant, and survey scan speed."
This, for me, accounts for the CA and all personnel and material release limits.
Will I find that, just as obscure, the origins of "non-detectable after 7 half lives" rests in a moment of simple necessity, rarely questioned.
"Every science starts as philosophy and ends as an art."
Will Durant, The Story of Civilization