To open the doors at the bottom when it stops, target them with the last path_corner.
On the way up, target them with the top path corner.
Another way to do this is use a trigger in the elevator, but that involves a bit of trickery getting it right.
Walk into elevator, into trigger brush... doors close.
Press the button, Elevator goes down, another trigger brush is at the bottom of the shaft, when the player goes through the trigger, the doos open.
Player walks out.
Trigger_multiple is good for this with a reset of 10sec.
Or you could go all out and use globals and multisoruces?