i would say spoiler but at this point you probably want it
on those unpickable doors, once you have the "key" you don't need to pick the lock or use the item; if you have the key the door will open normally. example: just east of the duergar room and north of the cambion is the door to the air elemental plane (airplane
). this door is unpickable. its "key" is a statuette found in irenicus' bedroom (east of the otyugh room, north of the 3 dryads). when you have the statuette and open the door, the statuette will disappear. some keys will do that.
good luck.