I love the idea of flying into Portland, ME, renting a car to visit Portsmouth and the Wentworth by the Sea, and then returning the car to the Portland Jetport and taking the downeaster all the way back to BON (north station). If you are going to take the bus fromPortsmouth to Boston, I would recommend C&J trailways over the amtrak bus - C&J a friendly, clean service with multiple departures per day. You can also look into the Concord Coach which has bus service from Portland, ME to Boston on a fairly regular basis.

Have a great time - the Wentworth is really a beautiful and spectacular hotel - as is the Custom House.