I would have to agree with Jeff Lubell, finding a realtor who is an expert in all of those jurisdictions will be a challenge. A realtor can sell anywhere in the state of Virginia, and get the job done, but finding someone who "really" knows all these areas will be the hard part.
I am also a native and grew up inside the beltway and have sold homes out in Loudon Co, as well as Madison Co, and can share that there is a world of difference between selling a home there and selling one in Arlington.
In reading your posts, my thought would be to suggest that your wife and you narrow your search a little more and then zone in on an agent who really sepecializes in the area you seek. Aoso, I forget the exact figure from The National Association of Realtors, but it is something like 87% of buyers and sellers find their realtor thru the recommendation of a friend. Good luck in your search, and if you find your focus is Arlington I would be happy to help.