Elite Interceptor // Rejoinder

Casting a copy of a prepare spell from exile isn't casting it for an alternative cost. Effects that allow you to cast a spell for an alternative cost or without paying its mana cost may allow you to apply those to a copy of a prepare spell cast from exile.

A creature with a prepare spell can't become prepared more than once at the same time. For example, Emeritus of Ideation is on the battlefield and is prepared. When it attacks, its last ability triggers. When that ability resolves, its controller may still exile eight cards from their graveyard, but doing so won't cause Emeritus of Ideation to become prepared a second time or create a second copy of Ancestral Recall in exile.

As an effect causes a creature with a prepare spell to become prepared (including effects that state that a creature "enters prepared"), that creature's controller creates a copy of that creature's prepare spell in exile. That copy remains in exile for as long as that permanent is on the battlefield and is prepared. That creature's controller may cast that copy as long as it remains in exile. As they cast it, that creature stops being prepared.