The trapdoor in the elevator shaft should drop you all the way down to a lower level. Have you tried switching to a smaller race (ratling, gnome or pixie)? Often they can still move where the big guys get stuck.
Later you will be able to use the elevator in the shaft to get up. Although I saw a message of someone who had stacked enough crates to be able to reach the next higher floor. This might be a solution for you too if you're keep getting stuck. That way you skip nothing that prevents you from completing the game, but of course you'll miss some of the story, xp and useful items.
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