-If the BOS-MTR plan goes ahead, a connecting, compatible line from that line to New Haven might happen to satisfy the NYP-MTR market demand (since otherwise, you'd likely get some wacky extension of the shuttle to save people going to Boston and then effectively backtracking...cue a royal mess)
-More likely, I think NYP-MTR would go up the Adirondack's line. My reasoning is this: 1) Albany is still a bigger market than most of the intermediate markets on the Vermonter, NHV possibly excepted; 2) IIRC, there is some kind of plan to upgrade NYP-ALB, but I can never recall what the plan is or what the desired equipment types are; 3) NYP-MTR avoids the Connecticut-related hangups that make a mess of NYP-BOS service at times; and 4) If the NYP-ALB equipment types are compatible with the Acelas, Amtrak might simply (with a large enough Acela II order) run a couple of the terminate-at-NYP trains up the Hudson to ALB, and then continue a few to MTR (while others, again pending compatibility, would go out west in NY state).
Another question: How long would a restored Montrealer have to go once it hit the border? If it's only 30 miles or so, just "putzing" along for that last segment might not be a deal-killer if it's "only" 30 miles.
Finally: Where can I get a look at NY state's plans, afigg?
To your last question, google is your friend.
New York
2009 State Rail Plan (Page 92 mentions a 2004 analysis on reducing Adirondack Schenectady to Rouses Point trip times by 38 minutes).
Massachusetts
2010 State Rail Plan
I suspect you are over-thinking the current plans. MA would be looking at a BOS to Springfield to Montreal train (with track and signal upgrades from BOS to SPG). Which might meet up with a NB Inland Route Regional at Springfield, but this would the even slower route to Montreal for someone in NYC over the Adirondack.
VT wants 2 daily trains over the Vermonter route in VT. The 2nd train may be the BOS-MTR train which would provide direct service from the VT stops to Worcester and Boston. Now if the Vermonter is extended to MTR, that would provide one seat ride to MTR from WAS and PHL for those willing to take a long day trip and an alternate one seat ride over a longer route from NYP.
We are a long way from running any Acelas up to Albany, let alone to MTR. I don't know if NY State and Amtrak have ever done serious engineering studies of catenary electrification to Albany. Now that Amtrak and Metro-North have control of the line to Albany, that might be on the table, but the NIMBYs along the Hudson river would present a considerable obstacle. How Metro-North would regard catenary over their portion of Hudson Line, no idea.
What NY State discusses in their plan is cutting the Albany to Montreal trip times to 6-1/2 hours which would require pre-boarding and arrival customs checkpoint in Montreal, not a hour plus stop at the border. If the Adirondack NYP-MTR trip times can be cut to 8-1/2 to 9 hours, that might get enough ridership to support adding a second daily Adirondack. One that departs early in the morning, the second at mid-day. That is how support is incrementally built for a true HSR line from NYC to Montreal.
If there were 3 daily trains (Adirondack, Vermonter, BOS-MTR) from the US to Montreal, that could provide enough of a base to justify a secure customs facility setup in Montreal. Three daily Amtrak trains to Montreal would be very cool. All daytime trains however from my read of the plans.
