I can't help you with passenger lists, but you should know that the Mauretania was used as a troop ship during World War 1. I don't know exactly when she was returned to civilian service but it wouldn't have been before January, 1919.
My grandmother, an army nurse, returned home aboard Mauretania in December 1918. They landed in NY City at the very end of December - she told me that she spent Christmas Eve on the high seas and New Year's Eve in New York.
Dianne in CT