The only way I see to keep going to Central Station from the CP is to continue up to St Luc yard, loop around to the west 180 degrees, slide over to the CN St Laurent sub, and make a left onto the Montreal subdivision. I don't know if switches exist from the CP to the CN adjacent to St Luc to do that.
But I think it would be far more pleasurable and productive to just get CN out of our lives and head to Lucien L'Allier. I was in both Montreal depots in July. Lucien L'Allier is fine for our purposes and has more waiting room seats that are not broken like the cheap plastic ones at Central station and 2 EXO ticket agents at this rather sleepy terminal and the Adirondack is not there rush hours. Metro stop is way down beneath it via 3 escalators. Arrange with EXO to haul the train back and forth to Montreal West yard for cleaning and turning as with Toronto's Mimico. The 2 EXO agents can be give an Amtrak computer terminal to do some rudimentary ticket processing like NJT agents do at Princeton Junction.
It would also be nice not to deal with stairs heading down to the tracks. Central Station personnel are very stingy about having people use the down escalators unless you are a person in need. There is no first class lounge, checked baggage, nor red caps, so I don't get the lack of amenities argument. There are no fast food eateries there, but so what. Montreal is like Manhattan - you can't walk a couple of hundred feet down the street and not walk into one.
Unfortunately, I don’t think either Amtrak or NYS-DOT over 3 months of this have given these options any thought, just continued brooding about big bad CN. But they and the local politicians will waste no time in putting on the fake shocked and outrage routine when the train gets the axe again next June.
There is no easy way to incorporate Vermont service into it, but that is IMHO a fantasy. It not only requires CN upgrades as far as Cantic, serious upgrading of the CN Rouses Pt Sub, training Vermonter crews in French literacy, Canadfian railroad regs, battling CN unions to not drive the train north of St Albans, and a Customs facility be constructed in Central Station, which nobody in Canada is serious about doing, just award millions to contractors to repeatedly play with their crayons and coloring books to plan one.