If you do leave the C&O in Brunswick, I would take 287 only to Lovettsville, then jog west on 673 and get onto 690 (mountain road). Thats a much more lovely road to take to Purcellville (I have a few road rides that takes this route and its very nice and almost entirely car-free). Follow 690 through the jog in Hillsboro (you'll be on route 9 for a few hundred yards), and when you get close to Purcellville, take a left on Alder School Road. Follow that to a right on Purcellville road, which will take you right into downtown and intersect the W&OD. Much, much nicer than riding 287 the whole way.
As for south of Leesburg, I can't help you there. But 15 isn't really a spectacular cycling road, from the sections I've seen. If you are willing to come closer to the DC, you could get off onto 7100 (Fairfax County Parkway), which has a decent adjacent bike path running almost the entire way south. Take it to 123 (which also has a path) and you should be able to pick up the ACA east coast route somewhere (not sure where it intersects though). I will note that these roads aren't exactly scenic, and the roads themselves will definitely have heavy car traffic (which you'll be separated from, but the noise gets annoying).