X-ingway is an Eidolon.He's able to go anywhere, with no indication of how he does it. He is seemingly immune to attack by regular monsters. He talks to Whyt, an Eidolon, and Fat Chocobo, who might be one. Finally, he gives you Augment abilities that are related to Eidolons (????'s Love, related to Whyt; and Safe Travel, an ability normally the province of Chocobos, one of which is an early Eidolon for Rydia). To finish the quest for above, you must fight him, just like Bahamut. Essentially, he is a bridge between the summon realm and the world of Final Fantasy 4. That's why he can't be summoned.
The Rabbit Hummingways replaced the Gnome Hummingways through Time TravelObserve Exhibit #1◊ and Exhibit #2.◊ The Rabbit Hummingways were a species of Time Traveling Cuckoos. They replaced the "Turbaned Gnome" Hummingways; and nobody "remembers" the original.
Everybody is an orphan/ Only orphans can save the worldCome on: Name the characters in this game, who have parents on screen! And now, count, how many of this parents actually SURVIVE the game. Not many left, hm?
Turning yourself into a monster made of evil actually makes you weaker.Think about it. Back as Zemus, the villain effortlessly shrugged off some of the most powerful moves in the game, being killed by an attack that does 99,999 damage. As evil, he was a tornado, a hairball, and a bug, each time being vulnerable to things like Meteor or Firaga.
The Darkness used by the Dark Knight is not evilBut rather, the use of the equipment and the dark power makes one more susceptible to Zemus's control. Note that, as a Dark Knight, Cecil's MP, Spirit, and Magic Defense are all abysmal. His Spirit stat will never increase past 1. Note also that all of those stats shoot up when he accepts the light given to him by Klu Ya. The Darkness isn't a corruptive force that leads to inevitable evil; it and the tainted equipment (which, might I add, is the only equipment available to Dark Knights and is only available to Dark Knights) combined drop a persons mental defenses to almost nothing, allowing Zemus to take control far more easily. This also gives a possible explanation as to why Cecil is so comfortable with using the Darkness again in Dissidia: Final Fantasy; his time as a paladin has bolstered his mind a lot, so he can safely wield the Darkness again.
Leviathan sank the ship specifically to grab RydiaRydia appears to be the last summoner, and we've seen a demonstrated link between summoners and the summoned. Leviathan was afraid that, or foresaw that, she was too young to survive the fighting she'd have to endure, and 'ate' her specifically so she could get her Plot-Relevant Age-Up. Stranding Cecil so close to Mysidia as to get Paladin'd was either part of the plan or a happy bonus, and the other casualties were...well, he's the king of the Fey, so he doesn't have to be nice about this sort of thing. Relatedly, he wants Rydia to rejoin the human world more in The After Years so that she can have children, and prolong the summoner bloodline.
Cecil and Rosa were adopted by Firion and Maria, just as Cain was.With all the hints that Cain from II is the same guy here grown up, this makes sense.
Emperor Mateus was brainwashed by Zemus.He was Zemus' first attempt at getting someone to gather the crystals. Unfortunately, he didn't count on Mateus' thirst for power. Then he heard about KluYa's sons, and decided to try and brainwash one of them, figuring they'd be easier to control being half-Lunarian.
Cid is the same person in both II and IV.Continuing the chain of II and IV connections... this one is a bit more wild mass guessery. As Cid died in II, he couldn't be the same Cid in IV... but then you consider the fact that Cid /should/ have died in IV on more then one occasion... So perhaps he found a way to cheat death and escape the afterlife? Then he really /did/ die when he fell... and just reentered his broken body again.
Golbez's dual heritage served as an advantage to Zemus.It is stated that Golbez's Lunarian blood made him more vulnerable to Zemus' control. Thus, it seems the similarity of heritage factors into the effectiveness of mind control. Why couldn't this apply to humans who use mind control? Maybe humans with mind control abilities find it easier to control other humans. So by taking control of Golbez, Zemus would both have someone that he could easily control while enhancing his ability to control humans through Golbez (i.e. Kain).
All the characters in the Tower of Prayers at the end are Dead.All of them, every one, suffered life-threatening situations that they somehow miraculously "recovered" from and subsequently "reappeared": Cid, Yang, Edward, Palom, Porom, etc. The Troian Clerics could have perished due to prolonged separation from their Earth Crystal, while Giott could have perished in the battle with the Babil Giant. The Master/Elder could be channeling them somehow and sending their spirit's wishes to the Moon...which would explain why Tellah appears to give support in the final battle as well(though doesn't explain why he isn't in the spectral assembly at the Tower of Prayers). Even the Elder himself could have perished by the crushing wall while trying to cure Palom and Porom of their petrification.
Leviathan is a Chessmaster.When he sank the ship, he didn't do so just to grab Rydia and everyone else survived by sheer luck. Instead, everyone ended up exactly where he wanted them to, where they needed to be. Cecil ended up near Mysidia, where he could atone for his earlier misdeeds and gain the power of the light. Edward ended up in Troia, where he could be close enough to the Dark Elf for his song to be effective across Whisperweed. Yang ended up amnesiac at Baron, where his strength would let him earn their trust and thus provide a way into the castle. Leviathan used his control of currents and the seas to make sure they all drifted where they did, because he knew this is where they needed to be, acting either on information from Bahamut or that the waters of the world brought to him. (The crew of the ship, possibly, he just let wash up somewhere safe and we just never see them.)