As I recall, Amtrak had a couple of ticket windows in Montreal. I think that they were actually wearing Amtrak uniforms, but they might have been in VIA Rail uniforms. Not totally certain on that point.
You will however have to use a VIA Rail redcap if you want help with the luggage and help for the person with mobility issues getting to the train.
I will note, although I do admit that I've not had extensive experience with VIA's Redcaps, that the two that I have dealt with were quite happy to tell me that I wasn't tipping them enough for their services. I've never had an Amtrak Redcap do that, even though I'm sure that there have probably been times where I should have tipped them more for what they did. But on my one experience with the Adirondack, I handed over 2 bags and pulled out $5 and he looked at me and told me that wasn't adequate.
Finally, be warned that the line for this train starts growing easily 1 hour before departure and it can get quite long. Also of note, they generally park this train on a track that does not have an escalator going down to the platform, so one must go down a bunch of stairs.