While I was typing my reply, namida posted.
Since Lemmix was built from DOS code, I'm pretty sure it would work the same.
I don't think you answered his question fully. This might help:
The game(as namida stated) cycles through each lemming to check if it is in the trigger area. This means(on DOS at least) that the lemming's "coordinates" are in the trigger area. Those coordinates are the ninth pixel under the lemming's feet(3x9 on a graph). If those coordinates are in the trigger area, the lemming gets killed/goes into the exit. So for traps and exits the answer is number 3.
For one-way arrow walls the game uses the same pixel that it uses to check for steel to check if a one-way arrow wall trigger area is there.