Platforms 09/10 and 11/12 have a second set of stairs just north (left) of the great hall flanking the Second Cup, according to this pretty recent floorplan which I have marked up in red.
I had to turn this diagram upside down because it was the other way up from the CN track-level plan I posted previously. The red text on some of the staircases says "up" not "dn".
View attachment 294065
The stairs further down the platform are a bit more curious. According to the 50-year-old track-level plan, tracks 09/10 and 11/12 should have stairs on either side of Rue de la Gauchetière, and plaform 07/08 should have one staircase on the north side only. The present-day ground-level plan only shows the stairs from the abandoned platform 07/08, which are apparently accessed from one or more of these unmarked doors on Rue de la Gauchetière.
View attachment 294060
The stairs from 09/10 and 11/12 are nowhere to be seen on the ground floor plan, but there is a suspiciously staircase-sized unit marked in the parking garage right over platform 11/12. Sure enough, looking through the glass doors via Streetview you can see a railing heading down.
View attachment 294061
But the numerous "Entrée Interdite" signs make it pretty clear that this staircase is no longer in public use.