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.

If Guardian Idol has any +1/+1 counters on it, those counters will remain on it after it stops being a creature. Those counters will have no effect as long as Guardian Idol isn't a creature, but they will apply again if it later becomes a creature.

Activating the last ability of Guardian Idol while it's already a creature will override any effects that set its power or toughness to a specific value. Effects that modify power or toughness without setting them to a specific value will continue to apply.