Alright, I've read all of your guys' posts and thoughts, but I still think it goes down like this..
From the beginning of the game Zelda is sealed inside the crystal, and Impa has Zelda's bracelet. Therefore that sets out the theory that it's all predetermined that those events will, and have already, unfolded. No split time-line, just you doing what you would do, regardless. Destiny, set in stone by your own actions.
So, anyways. That sets out the idea that your events you make occur, are predestined. If you think about the end of the game, Fi says that Demise's residual conscience is sealed away in the Master Sword. Not destroyed, but sealed.
Then at the endgame you seal that sword in a pedestal in the past, where Impa, and sealed in the crystal Zelda watch over the seal. Yet, if Zelda is there since the beginning, then, the Master Sword must have been there in the future as well right? Right, and it was. Actually, my guess is that Impa took the sword, and created the sealing spike, using the Master Sword as it's center. Which explains why a Skyward Strike "charges" up the pedestal to seal it a little longer. Giving the sword a little more power extends the seals duration, no? So that's why the Master Sword is never really seen by you, even though Impa knew where it was. Also, since Demise needed to get to Zelda (who was sealed up in the temple) to restore himself to his former power. Impa, of course probably thought leaving the sword so close to the sealed Zelda was a bad idea. Rightfully so... Still kinda weird to think, you "killing" Demise in the past at the end game, was the original seal you had been restoring throughout the game. That also explains why you still needed the Triforce in the present to actually kill Demise. You sealed Demise in the past, and throughout time the Master Sword was losing power, so until you used the Triforce wish to kill him for good in the present he could've came back.
Long story short..The seal that you assumed was Hylia's doing? Was more than likely the Master Sword in the sealing spike augmenting and making Hylia's seal stronger. As you notice there was no sealing spike originally in the past. Impa was messing with stuff, and helped guide you to fulfill what she knew you could/would do.
That is what seems most likely to me, but if you see any holes or want to discuss, feel free.