You're not adding a NW quadrant to the Palmer diamond without a massive amount of $$$... you'd have to fill down in a gulley and probably start the track 1/2 mile west of the diamond itself, since the NECR line runs NW/SE.... its not a true N/S/E/W intersection. Last time I was on board the Vermonter in November, the direction change was accomplished in under 10 minutes. The combination of 15 miles of running on mostly single track iron on the busiest freight line in New England is the main inconvenience... not the reverse move itself. If you're going to spend the dough to build the NW quadrant, that $$$ would be better spent upgrading the Conn River Line.
Are RDCs/DMus/whatever the best idea for this route? Personally I don't think so... lots and lots of grade crossings which get backed up with snow in the winter. Today's P42s just plow right through with no problem. I've seen pics of RDCs coming into North Conway NH in the winter, being led by a diesel locomotive because they were "too light" to clear the crossings. On Valentine's Day 2007, we got some 20+" of snow, which the NB Vermonter just plowed right through with no problems, and you're not doing that with a RDC/DMU.
When I first moved up here, I heard from various people who missed the Montrealer, as they'd take a day trip, leaving in early morning, returning in the evening. Never did I realize before that it was that popular with that crowd. Now that I live up here, our closest city is Montreal, and many do miss the train. Besides shopping, etc, any sort of entertainment - concerts and such, the closest is Montreal. Heck, if there was a train to Montreal, I'd ride it... I've never been "over the border" but I've come extremely close.
I wonder how an intermediate solution of reducing frequency would work.... operating only Thurs-Sun during certain times of the year. October (foliage season) this year produced very high ridership at my local station. November is up as well. Maybe another solution would be to run only 3 Amfleets + 1 cafe/bus class, eliminate some of the local stops in Connecticut. Have a 400-series "shuttle" running on its heels to do the local business. Many use the train as just a plain 'ole corridor train from WAS to SPG. At least we only support the train's operation north of SPG.