The Seanchan prophecies don't just state that - they go further to suggest that the DR will fight the DO "for the glory of the Empress" after she leads the fight against the dark. This does not make seem likely in light of the Seanchan's total lack of interest in fight the dark - they are commited to subjugating the Light's forces instead, channelers in particular. Sending assassins to murder Aes Sedai, who'll be desperately needed in the Last Battle, is just one example of many.

However, Fortuona is determined for this to take place, so I believe that the Seanchan will use their newly acquired Traveling ability to bring the Crystal Throne to the mainland and then try to capture Rand and make him kneel (using the Domination Band or the forkroot tea). I expect the Crystal Throne to be destroyed in the struggle - which will be the proof that the Seanchan prophecy is false. This will lead to the fulfillment of the mainland prophecy where the Nine Moons will be bound to serve the DR.

For me a strong indication why the Seanchan prophecy should not prevail is that their empire is founded on evil and slavery. How can such a force lead the fight against the Dark?