A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.

Guardian's effect doesn't wear off.

After the triggered ability resolves, Opal Guardian is no longer an enchantment.

A face-down creature spell will cause Opal Guardian's ability to trigger (if it's an enchantment).