The bickering of the spells is cleared up somewhat by the creation passages on pp. 103/85-119/99 from Eric. It is quite clearly stated that first the Creator did an Egg and Cress (for Rincewind), then He Cleared His Throat, then He Read the Octavo (that’s the word then), which created the world and finally the primordial slime came into being because Rincewind couldn’t eat the Egg and Cress Sandwich and just dropped it on the beach. The Creator subcontracted for the firmament, so it isn’t quite clear when that came to be.
“In the beginning was the word” is of course also a biblical allusion to John 1:1: “In the beginning was the Word, and the Word was with God, and the Word was God.”