There is one Amtrak train a day that travels from RKV to WAS, and it's a long-distance train that's frequently late. If your goal is to avoid driving into DC and paying for parking at Union Station, you're going to be better off 1) taking Metrorail from Rockville to Union Station and starting your Amtrak journey there or 2) finding somewhere to park in Alexandria, Virginia, and starting your Amtrak journey at ALX, which has regular northbound service through to NYP. (Starting your trip at New Carrollton in Maryland would also work, but I realize that's a longer schlep for you from Potomac Falls, and also the parking options are better around ALX.)
Alternatively, if you're flying back anyway, why not use BWI Amtrak/BWI airport, stash your car in long-term parking, and not have to worry about the logistics of reuniting with it later?
To answer your other questions:
Yes, you must buy by segment.
No, there's no real benefit to buying Business if you're riding the Northeast Regional. (I don't know what the rolling stock on the BON to POR bit looks like.)
Yes, all the trains will have Wi-Fi, although don't expect consistent signal strength or connection.
Last edited by ambyr; Mar 1, 2018 at 3:54 pm