When is the determination made on whether or not Mysterious Island effects will be used? Based on the Setup rules it seems it would be decided between placing islands and placing terrain, but before choosing home islands.
There’s not really any specific timing on that, but it makes the most sense to decide whether or not to use them before you start. At the minimum, you should decide before players choose the islands to include in the game, because allowing or disallowing MI effects will definitely influence which specific ones get chosen, regardless of who actually places them or where.