Where do those other "vertical access points" come out in the great hall? I don't ever remember seeing them.
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".
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.
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.
But the numerous "Entrée Interdite" signs make it pretty clear that this staircase is no longer in public use.
I don't see any traces of the stairs from platform 09/10. If someone's in the area, they could try running at the wall between tracks 09 and 10 to see if they pass through, but barring that I think we can assume that they've been demolished.
I have no idea what the fate is of the staircases on the south side of Rue de la Gauchetière, because the floorplan doesn't extend that far and I don't see any clues via Streetview.
As you said, platform 07/08 had its staircase removed to create more retail space (I beleive Bureau en Gros, aka Staples, occupied it last time I was in Montreal), so using it would require convincing the owner of the station, Cominar REIT, to evict a one of its tenants.
I have marked the position of the former staircase on the plan above by extrapolating the dimensions of the other staircases. The location of the row of pillars (in the centre of the platform) confirms that this is the right position.
The staircase itself doesn't actually encroach on the Bureau en Gros, but the store would need to be cut back to allow people to access the staircase. Evicting them would be unnecessary.
But in any case, I never said that we would reinstate that staircase. In the concept I described, there would be a large platform between tracks 07 and 10, so people could access track 07 from the current platform 09/10 staircases and elevators.
Also, if you look at this track diagram (not to scale) tracks 4-12 are completely isolated from tracks 13-23 until they merge at switch 195. I beleive the plan is to break that merge and run new set of parallel tracks out of the station for the REM. Reconfiguring the tree to allow VIA access to tracks 11 & 12 would require more work. You can actually see the construction for the new bridge across the Peel Basin in Google Maps (also below).
Yes, like I said, it is clearly much easier to use 09-12 for REM, since it maintains pretty much the existing track configuration, with a pair of tracks each for the Montreal Subdivision and Deux-Montagnes Subdivision (now REM).
But my concern is that once the megaplatform is built between 09 and 12, it will be quite a challenge to relocate in the future if we ever want to run additional train service into Gare Centrale. 10 tracks may sound like a lot, but terminal stations have a notoriously low capacity per platform due to the long dwell times and conflicts in the rail corridor.
Regardless, this change will help VIA a bit, since they will have fewer trains to compete with when approaching the station.
The only trains that REM is directly replacing are the Mascouche and Deux-Montagnes trains which never conflicted with VIA in the first place since they exited from the north side of the station. VIA's own Jonquière and Senneterre trains could theoretically get cut back to Côte-des-Neiges REM station and not serve Gare Centrale, given that transferring to the REM would be considerably faster than the huge detour VIA trains need to take. But I'm guessing the trains themselves need to be downtown anyway to go back to the yard, so they'll continue serving Gare Centrale.
One comment about your second map. I believe platform 21/22 is reserved for EXO's
Mont-Saint-Hilaire and
Mascouche lines, and VIA will use tracks 13-20.
I was wondering about that. 21/22 does make sense for Exo considering all their trains will be heading to/from the east once the REM opens (the Mascouche line will be cut back to Côte-de-Liesse station). There is also another staircase on that platform into the great hall that I had missed when I marked up the first map in my previous post.