Put customs off the train in one spot where resources can be shared, and run multiple daily trains from there across the border.
The Vermonter/Montrealer shouldn't (opinion, not won't, fact) have any issue using the same customs staff as the Adirondack, when that pre clearance facility eventually gets built. Hopefully once it's established we can get a night train or two rolling on the Montreal to DC via NYC run.
I do think for the foreseeable future having new through trains continue past the first Canadian station isn't feasible (and likewise for a hypothetical Via train connecting on the US side going past a station), so a Montreal to Chicago via Detroit trip will need connections and can't reasonably be an overnight one seat run, and the Maple Leaf is going to continue to be an issue.