[Now that was a whole lot of information to absorb at once. But she's Twilight Sparkle, she can totally handle it... Even if a lot of this seems utterly bizarre.]

Okay, so let me see here. I think I'm going to have to tackle these one at a time. Let's start with the idea that there's a limited amount of magic in your world. If that's the case, what's the source of the magic? It's clearly not human beings themselves, or the magic would grow as you did.