I'm in agreement that the Ocarina seems to be a key, but one kept as a safeguard by the Royal Family. That way, one could not enter the SR without their consent, even if they suceeded in acquiring the stones. I would suspect the Ocarina was created for that very purpose when the Door of Time was created. The Master Sword seems to have been the 5th requirement, and probably the hardest to fulfill.

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" I am the Hero of Time. No matter when, no matter where... for the sake of Hyrule and you, Princess Zelda...
I shall fight." - Link ( OoT manga )

Articles/Theories:
The Case for Kingship ~ Light and Shadow: The Tale of Two Princesses
Majora's Mask: Parallels in Love