What I was thinking was, you'd enable DEBUG_MODE (line 5 of init_constants is "#macro DEBUG_MODE false", change it to "#macro DEBUG_MODE true") and then you'll get messages telling you what happens. E.g. if a door transition takes priority, the game will print "Came through a door, jump to LABEL" (so if you're not where you think you are, you can check that the label is correct) and it'll tell you "Player loaded! (room=NAME OF ROOM)" as well so you can tell where you ended up.
You could try adding additional show_debug_message's to e.g. print the player's position after loading, that should make it easier to find in the room editor:
show_debug_message(tsprintf("Player location: %, %",x,y))
As I said before, going through a door will place you at the door with the same name in the other room. So a drawback of this approach is that you can't link two doors in the same room together (because when looping over the door objects it'll find whichever of the two doors with that label is first in the instance list and always pick that). If you want to warp between places in the same room I'd probably create two new objects, "obj_teleport_entrance" and "obj_teleport_exit", which has a label the same way the doors have. Then in player's collision with teleport_entrance, run this code:
var target_label = other.label;
with(obj_teleport_exit){
if(label == target_label){
other.x = x;
other.y = y;
break;
}
}
Now each teleport pair would use one entrance and one exit. If you want a teleport to be in both directions you'd have entrance A and exit B on one side and entrance B and exit A on the other side; make sure to place the exit some distance away from the entrance (otherwise the player gets stuck in an infinite loop of warping between them).