The one thing that seems weird about the ideas floated for resumption of US-Canada train service through Detroit is that they seem to favor sending Amtrak trains to Windsor, as opposed to sending VIA trains to Detroit. It would seem that the latter would make more sense - more people are traveling between Toronto-Detroit than Chicago-Windsor, so it would seem you would want the former as a one-seat ride. There’s also considerably more options for intermodal connections in Detroit.
Furthermore, it would seem like you could run VIA into the existing Amtrak station (or a new station on that line) by the connecting track if that was preferred. Of course that would eliminate the need for MCS, though you could still reroute trains there if you wanted to make part of that the new intermodal station. Best I figure it must be because Amtrak is the party interested in extension, and/or due to US PTC requirements. Though Amtrak manages to run service into Canada…
The proposals I've seen were Chicago-Toronto via Detroit. Detroit/Windsor, while a major traffic point, is where the border would be crossed, not a terminus. The model would probably be the Maple Leaf, not the Cascades or the Adirondack. That is because it would have many stops and local service in Canada between Windsor and Toronto. That means it would be operated by VIA as a VIA train in Canada and by Amtrak as an Amtrak train in the US, irrespective of whose equipment is used. Like the Maple Leaf, it would be a one-seat ride, albeit with detraining for border inspections.
VIA, the deceased Atlantic Limited across Maine notwithstanding, appears totally allergic to operating outside Canada. All cross border services are operated by Amtrak. Even on the joint Maple Leaf, it is an Amtrak train as far as Niagara Falls, ON.
I understand the International ran, at least for awhile, with one VIA trainset and one Amtrak one. It also operated on the Maple Leaf model, an Amtrak train Chicago-Sarnia, a VIA train Sarnia-Toronto, regardless of equipment. With PTC requirements today, I imagine at least an Amtrak lead unit would be required, so the service, like the Maple Leaf, would likely utilize Amtrak equipment. So, like the Maple Leaf, PTC should not be an issue.
One major issue for such a service is not MCS, PTC, or which organization physically runs the train across the border. It is the geometry of the rail network in Windsor and the Windsor VIA station location. Accessing the VIA station from the tunnel would be indirect, slow, involving low speed connecting tracks and backup moves. There are not any good connections between the CP Windsor Sub and the CN/VIA Chatham Sub.
The western 50 miles of the Chatham Sub into Windsor are actually owned by VIA, apparently CN didn't have the traffic to support it and unloaded it. Since VIA actually owns the line, I doubt that they'd want to move onto the CP Windsor Sub even if CP would have them. Somebody is going to have to pay for a relatively fast, good quality connector between the VIA Chatham Sub and CP Windsor Sub. Hint, it isn't going to be VIA.