Southwest is one of the only airlines that will allow you to re-book if your fare drops. You will have one year to use the credit.
As for deals, if you haven't already, sign up for DING!
http://www.southwest.com/ding/what_is_ding.html?int=HOMEBNR4ADING00100127
Have you looked at other airports? I didn't think BOS or PVD was too far from MHT if the fares were worth it.
I will say that given your dates, I wouldn't be very optimistic but you never know.