Hi!
I live off 95 in Northern VA and I've driven the route you describe!
My advice would be to avoid 495 and get on 95 as far south of DC as possible. Traffic is impossible around here!
I would stay on 15 past Frederick, MD - take it all the way into VA, through Leesburg, past I66, stay on until it joins with US 17. Take 17 east and get on 95 there.
This would put you on 95 a few miles north of Fredericksburg. Traffic frequently backs up from 495 to Fredericksburg (no, I'm not even kidding) so even if it backed up when you get there it will break up south of Fredericksburg.
15 is 4 lane from Harrisburg, PA to the VA line. After that its 2 lane - except around Leesburg, but it's a good road. I've driven between 66 and Harrisburg many times! I haven't taken 15 S to 17 though.
If you must take 270 to 495 do it either mid day - 10am - 2 pm - or in the middle of the night.
If you hit traffic you could easily double or triple the amount of time you expect to take! Yes, it's *that* bad.
Good luck!