I can help with the second part, having ridden the Blue Ridge Parkway, its northern continuation--the Skyline Drive, and the western suburbs of Washington every year since 1976. I would leave the Skyline Drive at its northern end in Front Royal, then take local roads to Purcellville. This is rural riding in "horse country," easy and lovely everywhere.
From Purcellville, I would ride the Washington and Old Dominion rail-trail conversion (everyone calls it W&OD) all the way to downtown Washington traffic-free. Except, of course, at highway crossings, but the big ones use bridges or underpasses.
PS I have moved your related route questions to join this topic in Routes, where more coherent replies are likely, as people read the whole story. F