Forgive me that this is long. I spent a while thinking about the different angles, patterns, and dead ends. I followed the evidence given, and tried my best to logically explain how I think it all happened using the best probability:
The Doctor wrote:I always thought Celestia's actions in that first episode were just a hail-Mary play. Celestia had imprisoned her sister for trying to bring about the Ponyacalypse. She defeated NMM/Luna on her own, and NMM/Luna would not accept, or ask for forgiveness. Fast forward 1000 years later, and Celestia knows she is going to have to defeat NMM/Luna again, she also knows that she represents that which NMM/Luna resents. But, Celestia wanted her sister back. I think she gave Twilight the book to lead her down the right path, relinquished her control over the elements, and sent her her to Ponyville in the small hope that her sister could be saved. Celestia knew the only way to get Luna back was for someone else to defeat NMM.
The risk Celestia took depends on how powerful you think she is. Do you think she could have defeated NMM without the Elements? Were the Mane 6 in danger? I think she could have, that her concern was never about defeating NMM, she knew that could be done, her concern was getting Luna back.
Have an Egophiliac visual aid.
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Actually, that's an angle that didn't get covered before. Even if Twilight and company failed, could she have still defeated her sister without the elements? Assuming she could, it does solve that dilemma of the world forever being in torment due to a reckless gambit. I suppose the only question left then is were Twi and co. in danger? Well, there are a few avenues to go down with this road. The two dangerous happenstances were the cliff slide, and the manticore. Twilight could have easily kept sliding, slipped, and fell a very long way. The manitocre could have eaten or stung a party member. The question that arises here, would a member have actually died/become severely hurt or would cartoon physics take over? It's hard to say really. Evidence exists for both sides.
Hypothetical: If the answer is yes, cartoon physics would come into play, that still seems a bit mean of Celestia, but I could forgive her since Twilight would have embarked on this adventure regardless of the hidden masterminding. Nobody is in any
real danger. I don't like this method though because it takes away a large impact of the entire arc, and any real consequence in the world.
If the answer is no, they really could have died, that doesn't look very good for Celestia to put innocent ponies in harms way for her own bad blood. Before I get jumped, I'm not saying she's evil or a tyrant again, I'm just saying it kind of puts a wrench in this entire line or reasoning.
I suppose the only real out at this point is: Celestia was watching, waiting to swoop in and save them all with magic if it got bad enough. She didn't before, because she had an omnipresent view of the scenes (unlike the viewer). She knew Twi was going to be caught, she knew Fluttershy could defeat the manticore. It's
really really stretching it, but it does work.
So then, what's the last issue that needs to be covered? The 6 friends that happen to hold the exact talents needed for the elements of harmony. I'm not happy with "She just took a chance!" She had to have at least
some information on this before just charging in.
This is the 1000 year old Celestia we are talking about!
Not Rainbow Dash after all.
My final theory? Celestia rigged it so those particular ponies would be the main event organizers. That's the only link left, and it fits very well. Celestia did know about those ponies and their personalities, she
made Twilight interact with them. Everything after that, was up to her apprentice, who she had hoped was trained well enough as a good pony to complete the task.
TL;DR
-Twilight becomes the element of magic. Some time between her apprenticeship and NMM's return, a plan is concocted.
-Celestia surveys Ponyville for suitable ponies to wield the other 5 elements.
-She rigs it so they are the main event coordinators.
-Twilight is given the book and instructed to go to Ponyville with a subtle hint: "Make some friends!"
-Celestia raps up her duties, and bows out. Watching from a safe distance.
-Episode happens, we all know what transpires.
I apologize if this breaks some weird fan fiction rule. That's not my intention. I'm following the evidence given to discover the hidden motivations behind one of the most mysterious and interesting characters of the show in an attempt to present that her character is more than just a pretty pony princess cutout.
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Edit: 635 words, I think I have a problem or I really like ponies
:"Hey Rarity!"
:"Um, right. How tall are you?"
: 
It's a month or so until season 3.
What do you know about the burden of ruling over a kingdom? That's hard business, you know, being all nice and honest never works out because other rulers suddenly believe you've gone soft, and this often results in invasions and general unhappiness for everyone involved. And if you want to get anything done you have to use tricks and need to act all hush-hush, lest any opposition in the royal family or in the ranks of nobility might try to use the situation to their own gain, those backstabby no-goods.
