If an Aura is entering the battlefield and it wasn't cast or given a target, you enchant it to any object it can enchant. It does not target if it attaches this way.
Mental Meltdown overwrites the text of the spell, including characteristic defining abilities other than color indicators. It does not overwrite the card's name, type or mana cost, or any information on how the spell was cast, such as if it was kicked or cast for its prophecy cost. It also does not overwrite any non-text changing abilities acting on a card, notably, a card cast for its prophecy cost still can't be countered if it is enchanted by Mental Meltdown.