Yes, to reiterate, the Adirondack is a reserved train. That means that if you have a ticket, you have a seat. Now, the seats are not assigned ahead of time, (your ticket will not have a seat number on it), but you're getting on at the very first station. You shouldn't have any problems getting several seats together.
Heck, my friends and I boarded last year in Albany, several stops up the line, and the 3 of us were able to sit together and had plenty of open seats around us.
As for crossing into Canada, if you're an American citizen with no felonies on your record, you'll be allowed into Canada. It's essentially the same situation as it would be if you drove to Canada. In fact, it's the same border agents (they pull them from a nearby road crossing when it's time to screen the passengers on the train).
When I went into Canada aboard the Adirondack last year, the only people in our car who had any trouble was a Swedish couple who had decided that morning to buy a ticket on the train and ride up to see Canada. Since they bought tickets that morning, they weren't on the advance list given to the Canadian border agents. Additionally, they weren't Americans, so they were taken back to the cafe car to verify that it was okay for them to enter Canada. And it was fine. Everyone on the train was allowed to enter Canada.
As for whether you should drive or take the train, that's up to you. The Adirondack is by far the most scenic route on the east coast. It's an easy ride up to the Great White North. No driving stress, no getting lost, no paying an arm and a leg for gas.
Yes, in my opinion, you are worrying about nothing. After all, driving isn't problem-free either. What if your car breaks down? What if you get lost in the wilds of upstate New York? What if you get caught in a traffic jam for hours? What if the border guards won't let you across? What if you're in a terrible accident?
Travel comes with risk. For the most part it's a teensy tiny risk. I think your trip is much more likely to go smoothly on the train than it is by car.
And you certainly won't need your car in Montreal. The Metro system is excellent, and is very easy to navigate. The city also has a great bus network, and many of the attractions are close to Centre-Ville (Downtown), so you can get there by foot or by bike.
So, don't worry. Just let Amtrak do the driving for you. And enjoy the fantastic views of the Hudson River Valley, Lake Champlain, and the Adirondack and Green Mountains.
