I just realized, the whole "possessing the good planet's fighters" thing doesn't really make any sense. Just because the host is dead, we've seen that the Grandpa lifeform can exist separately, so there's no reason why it wouldn't fight off the Souboutei lifeform like it did inside of Rokurou. Even if it did die alongside its host, then it would lose all of its powers, so there would be no benefit to possessing the bodies. And in that sense, it still doesn't make sense how the Grandpa water would protect the Souboutei water from the air. What could it possibly do that the Souboutei water couldn't do on its own?