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).

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