Although I think these designations have changed, these are the descriptions I use:
MS60-MS62 - Uncirculated
MS63-MS64 - Brilliant Uncirculated
MS65-MS67 - Choice Uncirculated
MS68-MS69 - Gem Uncirculated
MS70 - Impossible
The proof coin will definitely stand out from the rest. As many have pointed out, grading is subjective and not everyone uses the same point scale to define Brilliant, Choice, or Gem. This may be why you ran into difficulties. PCGS will list a numerical grade. Dependent upon what you consider to be UNC, BU, etc. may not be the same definitions used by the dealers you approached.
Wouldn't it be nice if the ANA would grow a backbone and standardize the grading and terms?