Near the ending of the show, Lauren Faust published the following tweet
implying that Celestia herself released Nightmare Moon. A lot of fans of the show seem to doubt this though, because later when Discord is revealed to be Grogar, she admonishes him
saying, "You have made a grave misjudgement". This means that her releasing Nightmare Moon without a decent backup plan must have not been a misjudgement. To the audience, though, it seems very risky.
But was she really taking a risk?
From what we know, Celestia all by herself was not powerful enough to defeat Nightmare Moon. And with just the Elements of Harmony, all Celestia was able to do was banish Luna to the moon. As long as Celestia is wielding the elements and attacking her sister, Luna would view her as a threat, not as a means to reform. Theres no way Nightmare Moon would revert to Princess Luna all by herself. So I believe that Celestia's hypothesis was that if someone else powerful enough to wield the elements with powerful enough magic to weaken Nightmare Moons state, then she would be able to reform her.
Now, at this point in time Celestia doesn't know very many powerful ponies who can help her. She has enough magic to raise both the sun and the moon, theres not many ponies who have that kind of magic. The only ponies who she can ask for help from are
The only other alicorn known at this time, Princess Cadence. However, Cadence doesn't seem as magically gifted or knowledgable as Twilight is, and Twilight is more likely to overcorrect a problem she's not even sure about. Leave it to Twilight to freak out about something that likely shouldn't concern her.
Now put that together with what Celestia later told Twilight about Sunset Shimmer when she thought that she would have to send Sunset away. Celestia said that she chose ponyville for Twilight because she saw a very promising group of 5 other ponies in ponyville, ponies that were good friends. Considering how easily Twilight figured out how the remaining Mane 5 corresponded to the Elements of Harmony, I'm sure Celestia also realised this at some point.
But did Celestia even know what the 6th Element of Harmony was?
Did she know that it was magic? Or did she just know that she had 5 of them, and that because she was an alicorn, she'd probably have the 6th one as well?
Now this brings up more and more questions. Did she just leave out the 6th element of harmony because she wrote the book and thought that naturally curious Twilight would only understand if she came upon that conclusion by herself? Or knowing her mentor Starswirl the bearded, did she simply not know what that element could have been?
I believe that Friendship is more like the glue that bonds the 6 elements together, the spark, if you will. It jump starts the elements so that they work properly, but it does have its own kind of magic, because when these ponies work together, and grow their friendship, by the end of the series they don't even need the physical elements, because they embody the elements enough to the point that the differentiation between the physical amulets and the spirits of harmony is no longer necessary. Their friendshp has grown so much that its magic has surpassed even the Tree of Harmony and, ultimately, the final villain group.
What do you guys think? Does this theory hold up? Any holes that i haven't covered or that would keep this from being possible? Let me know! :33