I forget what chapter it was, but we learned that Fissival has a teleportation netowork, slightly similar to the Magical Door Erin has. However, unlike the door, it's not perfect, and a small percentage of things sent through it don't get there.

Now, if teleportation is this hard to get right, wouldn't it be really simple to intentionally get it wrong? Make it so that whoever activates the trap on the door just gets teleported a continent away or torn apart into constituent atoms?

Even if Teleportation can't be used lethally, why would it be so easy to reverse the enchantment? Why would the enchantment be able to have multiple destinations attached, if all it was created for was to send people into a pit of madness?

My proposal is that this door was originally built specially to be used for transport by people, as it's being used in Erin's Inn. Then for some reason (person who commissioned it couldn't pay, a better door was made, the city it was supposed to go to got destroyed, etc.) it no longer needed to be used for that. The Mage who owned those rooms instead altered the enchantments to turn it into the trap for their personal quarters, as the magical artifact was stronger then anything they could spare the time to make and it was easily configurable into the perfect trap (remember, just examining it with a spell was enough to trigger the teleportation). As all these modifications were made after the core framework of the spell, which meant Pisces (a skilled made, but not one specialized in enchanting) could then "untwist" the spell back to its original form.