To be clear, when you say "out of bounds with Priority pass via CSR", I don't believe this is an issue with the source of the Priority Pass membership, it's an issue with the setup of the airport. YUL is split up into 3 zones--domestic, US-bound, and non-US international. Both of the Priority Pass lounges are in the non-US international zone, and therefore cannot be accessed unless one is departing from the international zone. Someone in the Canada forum may be able to explain if it is possible to go into the non-US international zone when departing on a US-bound flight.