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

Guardian's effect doesn't wear off.

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 their most recent turn. It doesn't matter how long the permanent has been a creature.