It's the Eye of Ra. Yes, it is Egyptian. Truth be told, I was sleepy when I registered, so I was like, "I'm just gonna find something cool for that spot." However, to me, an eye represents intelligence and curiosity, so I may go with something similar whenever I change it.
It's very simple. You need simply venture to the moon, pick up a rock, find your way to Tartarus, present the rock to Cerberus, and he (they?) shall lead the way, as trained.
Yes. The flashbacks Twilight experienced were manipulated. What she didn't see was the Bananas of Harmony.
Basically. In a tunnel network deep under Canterlot. But they are protected by Princess Celestia's infamous twin, Princess Molestia. If she detects your presence, you'll learn the kind of trauma an alicorn horn can truly cause. And there will be no escape. So use a perception filter, and stick to the shadows.