Welcome on Urban Toronto and thank you for asking this excellent question!
I recall asking this question (why trainsets with one locomotive at either end did not still back out of Gare Centrale whenever they are used for one of the Quebec-Montreal-Ottawa trains) to my colleagues, and even though I don’t remember the answer, I suspect that a reason is that the reservation system doesn’t really support cars changing their orientation (or direction of travel) mid-journey, which would lead to either of the following situations:
- The car orientation would be shown wrong for half of the train journey, which I don't think is revealed to the passenger while booking online, but is probably an issue for customers booking over the phone and asking specifically for a forward-facing seat, as the phone agent would now have to identify 1) that the train has two locomotives, 2) that it changes its direction in Montreal, 3) which part of the passengers' journey would be before and which part would be after Montreal and 4) for which part of the passengers' journey the information displayed by the system is correct. This would provide ample of opportunities for human error on the side of the phone agent, which would inevitably lead to frustration on the customer's side. Also, it would force any passenger insisting on sitting forward-facing to book a different seat before or after Montreal.
- All cars would be loaded as facing one direction until Montreal and facing the other thereafter. Whereas this would work great with any passenger leaving the train before or in Montreal or boarding in or after Montreal, the system would now treat any passenger travelling through Montreal as a transfer (most probably assigning different seats for both legs, as it doesn't understand that it's the same car), which could of course be corrected directly by the phone agent when booking over the phone, but will result in two different boarding passes (with probably two different seats, potentially even in different cars) when booking online.
As you can see, neither situation would be desirable and I suspect that this is the reason why the entire train is turned in Montreal, even though the train would be perfectly capable of operating bidirectionally...
I hope this answered this (very first) question of yours and I'm looking forward to you making your nick name all honors...