If all other Auras attached to the enchanted creature stop enchanting it, Daybreak Coronet will be attached to an illegal permanent and will be put into its owner's graveyard.

If multiple Auras are entering the battlefield at the same time, you choose which player or permanent each enchants before any of them enter the battlefield. This means that Daybreak Coronet can't be put onto the battlefield enchanting a creature that's not enchanted before the Auras all enter the battlefield.