Here's a mock-up of the hold.
The area in the Yellow box is the #3 cargo hold. It's a 2-storey room, with the cargo shaft in the forward ceiling. The Fireman's tunnel is a single-level tunnel between the green lines. The room immediately aft (right side) is a coal bunker. Inside the coal bunker at the aft (right) end is the junction room with the escape ladder, which you can see rising up into the deck above. The room immediately forward (left side) is Cargo hold #2. In the forward part of the room (which is modeled here as a 1-storey compartment level with the Fireman's passage) is a small compartment for storing fresh water. It's likely the fresh water storage went all the way to the top (2-stories). The large room at the very right edge of the picture is Boiler room #6.
I believe there were large stabilizing fins built into the hull of the ship for rigidity, and columns supporting the ceiling, which are modeled here.
Here's a more distant view, showing the Fireman's passage through cargo hold #3 and #2. Again, the yellow box is around Cargo Hold #3
The green line runs the length of the Fireman's passage. On the forward (left) end of the fireman's passage is the enclosed, watertight casing which holds the twin spiral staircases.
Here is a view inside the cargo hold, forward port side, looking aft-starboard. We're standing on top of the water tank (actually, we'd probably be enclosed inside it - but in this model it's only 1-storey high). You can see the hole in the ceiling which is the cargo hatch. You may also be able to see, in the aft (far) end of the room, hiding behind the support column, just next to the Fireman's passage, the door into the junction room.
Here's a view from below the cargo hatch looking up (port side looking across to starboard). You can see the double doors leading to the mail room (lower and upper doors.) And you can also see the door to the specie room, just next to the small white cross in the center of the image. I put holes in the cargo hatches so I can move through them in this model the wall in the left lower corner is the water tank, which probably rose up to the ceiling.
Because the lower-class working crew had potential access to this room, and because the mail and specie room were directly above, I imagine there was no way to access the upper level, and especially not the cargo shaft while at sea. I don't believe there were doors leading forward to cargo hold #2. I think its very unlikely that the fireman's tunnel was 2 stories. There was no need for it, and it would have hindered the movement of cargo through the cargo hatch, which was directly above.
