In a Second Wave game, when can players play the Power Cards carried over from the prior wave?

In a Second Wave game, when can players play the Power Cards carried over from the prior wave?

Any Spirit Phase after the game has first entered Stage III.
If the Stage jumps around (e.g., against the Tsardom of Russia), the it does not matter if the game is currently in Stage III, so long as it has been Stage III at some point earlier in the game. (Note though that the early Stage III Card for the Kingdom of Brandenburg-Prussia does not put the game into Stage III.)
Since the Power Cards don't belong to any Spirit that's still in the game, it's impossible to play them any time other than the Spirit Phase, e.g., with "Spur On With Words of Fire" or "Ethereal Conjunction."