As you want back roads and you have a large, circuitous route I just mapped out the basics of the places you listed.
The Kancamagus Highway in NH is very picturesque but will be very busy in Oct especially on weekends. But, using your list here is a possible route for you: Portland to Conway, NH-60 miles;1 hr, 32 min; Conway to Lincoln, NH-36 miles; 54 min; Lincoln to Putney, VT- 114 miles; 2 hrs, 7 min; Putney-Shelburne Falls, MA-40 miles; 43 min. You would be about 1 1/2 hrs from Sturbridge Village if that interests you.
You don't say how long your visit will be but you have picked an arduous journey especially if you have limited time. Also, the back roads can be picturesque but will be slower than a highway(part of the outlined trip above is on a highway).
You need to decide and arrange housing NOW as northern New England is very crowded in Oct. By the way, from Shelburne Falls its over 2 hrs to Logan Airport or T.F. Green in Providence, RI. If you can leave out of Hartford, CT that is over an hour from Shelburne Falls.
Good luck with your arrangements have a wonderful trip.