Definitely no on the flying options, though they are good suggestions. Though DH won't admit it, he's afraid of flying - and he's a nasty travel companion while doing it. I prefer to avoid that if I can (can't avoid it on overseas travel, so I just grin and bear it )

Which place is better to change trains in, NYC or WDC? I think I've taken an American train once in my life (near Detroit) so I am clueless as to the safety and ease of transfers at stations

We would much rather rent a car than trust our car up there - it's used to flatlands, like Florida. I think it would croak if it saw a Maine Mountain!

We flew up to Maine once for Xmas/New Years a couple years ago, and got picked up at the airport, then drove another hour or so before we got near Dexter. We have no problem driving from Boston - I've never been to that part of New England, and my hubbie used to drive down for weekend drinking binges from college in Univ of Maine