Which Expo player who moved to the Nationals in 2005 remained longest (continuously) with the Nationals?
Comparing the 2004 Expos opening day roster with the 2009 Nationals opening day roster, it looks like Nick Johnson is the winner here. He joined the Expos in 2004, and stayed with the Nationals until 2009. I think Luis Ayala comes in 2nd place, as he stayed until 2008.
Livan Hernandez stayed with the Nationals until 2006, then came back from 2009-2011.