I don't have Eldridge in front of me, and haven't been through the canal since May, but given distance from Salem and need to time entry, it sounds like it will be tough to get from Salem through the canal on first leg. I agree with above to put in to Scituate and head out next morning. Maybe Plymouth, but it is a ways in and a ways out. Then, if you get a morning current ride through the canal and all things going well, get through Woods Hole (again, depending on state of current) and figure out whether you've got enough time to proceed to N'ket or stop in Vineyard Haven, and head out next morning. Cuttyhunk was suggested above, but it would take you well beyond Woods Hole and you've have to back track by a couple of different options to get back on your path. C'hunk definitely worth going to, but you sound like you want to get to N'ket. So, maybe with all that you get into N'ket on your third day, all things considered. Don't know how aggressively you want to get there. Eldridge will dicate what you can accomplish re: CCC, Woods Hole, Vineyard Sound, N'ket.
Hope you have a great time.