How exactly do those letters get into the mailbox, considering where it is?
That is what bugs you about the game world? Seriously?
Fandon decided the letters are delivered via Klaybird.
Where are those radio signals being broadcast from? From inside the 'Hood? If so, then who put them all together? Especially considering that The Neverhood seems to be a relatively young place. If they're from the outside, how close are they, really, to the nearest... er... Quatar-son-realm?
This troper thinks that the Hall of Records stated that Hoborg intentionally went pretty far from any of Quater's family because of his search for super-pure Klay. (which he used to make the Neverhood)
Who is Quarter supposed to represent? Jesus? Who do his seven sons represent in the bible?
Quater is most likely a sendup of Gnosticism. Gnosticism has the central conceit of a powerful deity who creates a lesser deity called the Demiurge. The Demiurge in turn creates the world that mortals dwell in. Within that metaphor, Hoborg is the Demiurge, with Quater representing the creator deity.