I believe Twilight's feeling is right on the money. That shield falling might be a prelude to the Heart's corruption being sent out beyond the borders of Canterlot itself, and in that case, Ponyville isn't a safe haven anymore than Canterlot itself was. That's not even factoring in what corruption Rarity might have unknowingly brought into town beforehand.
I believe Twilight's feeling is right on the money. That shield falling might be a prelude to the Heart's corruption being sent out beyond the borders of Canterlot itself, and in that case, Ponyville isn't a safe haven anymore than Canterlot itself was. That's not even factoring in what corruption Rarity might have unknowingly brought into town beforehand.
I do dearly hope this gets updated soon; it's a wonderful story.