1. It was the Darkness manifesting as Nimue. Right up until Hook summoned all the Dark Ones from the Underworld, Nimue was simply one of the many forms assumed by the Darkness.
2. Technically speaking, the Underworld and Hell are two completely different things. The Underworld is the afterlife of Greek Mythology, but is completely distinct from the notion of Hell we know as. The writers are simply using the terms interchangeably, which is incorrect. To Gold, the Underworld may have just felt like hell to him, but for all intents and purposes, it's not the same. The Underworld did have a place of suffering for evildoers called the Fields of Punishment, but it also had a "heaven" known as Elysium or the Elysian Fields. Lastly, neutral souls who committed equal acts of good and evil were consigned to the Asphodel Meadows, which was a sort of limbo.