It's not really a plot hole, it's just that we couldn't explore this thoroughly just yet. Hermes is the overseer, the deed as a physical object reverted to him. But he gave it to the old man, who requested it, and in turn he passed it on to the player character.
On my first Playthrough i thought the oversser must be Zeus or Hermes. However --as Mathmatican-- MC i answered "Athena" being the overseer for roleplay purpose. In most Saga i read (so yes, i am biased) Athena has been very helpful for humanity; Jason and the Argonauts, Odysseus, Thesseus and she propably helped Daidalos to contruct the Labyrinth. My MC assumed that Athena must be plotting against all Olympus to set Humantiy free from the Gods, therefore she set up the trail that Zeus was going against the oath to Styx.