Daniel,
I cannot immediately relocate the posting, but about 2 weeks ago Michael Standart posted a message about 3rd Class and Locked Gates. He referred to a paper in the latest issue of Voyage which I understand generally argues against the idea of locked gates as depicted in films. At least that is how I recall the post. Apologizes to Michael if I have mis-quoted him.
My understanding based on the a Report by Lord Mersey is that on
E-deck from the cross passageway; which was the forward 3rd Class Entrance that main alleyways ran aft on each side "through to the after end of the vessel". Although the port-side alleyway ran the full distance it was interrupted by some 5 water-tight doors. These were located at the the forward and after ends of the Engine Casing; just aft of the Turbine Engine Casing and at points forward and aft of the after 2nd Class stairway.
While I understand that those WTdoors remained open assuming they had been closed [manually, not from the Bridge] to have escaped from the forward section of the ship 3rd Class passengers could have used the ladderway [which you mention] from the forward Well Deck. There was a gate at the top; but as it was only rail height it could have been climbed over. The
deck plans in Eaton & Haas show two doorways near the forward 1st Class gentlemen's lavatory on
E-deck. What the "Escape" is and why the other door goes into one of the cubicles ..... Perhaps someone else can answer that? In the plans in the Shipbuilder [and therefore
Olympic] the Escape door is into a linen locker; with the other door going into a drying room.
Otherwise by way of a doorway into the 1st Class Entrance. I do not believe that it would have had any steps to it and doubt that it would have been a grill type doorway. Aft of that there was access via the Steward's Stairway. If those two were locked then the only escape route would seem to have been via the ladderway from the forward Well Deck; so it would seem that some 3rd Class passengers could have been trapped.
As for any 3rd Class passengers in rooms 101 to 126; if the WTdoors aft on E-deck were closed and they were denied access through to 2nd Class then it would seem that they would have been totally trapped. The same would have applied if rooms G-1 to G-40 were assigned to 3rd Class. Here and only here would there seem to be the possibility of a grill type dooway at the top of a deck high flight of stairs [as depicted by Cameron].
Others may have a bettter idea, but I understand that the doorways leading into 2nd Class were opened; but later closed.
I hope this helps,
Lester