Well, in Return of Harmony Celestia states that she and Luna were connected to the elements. Presumably both had half of them under their control, and they were inanimate objects during those days.
I theorize that Luna’s transformation into Nightmare Moon made her unable to use the elements under her care, and Celestia seized control of them. However, something prevented Celestia from using the elements to their full effect (inexperience with Luna’s half of them, or perhaps the emotional turmoil of fighting your sibling) and was forced to imprison NMM instead in order to buy time. She probably meant to set the timer a bit lower, but what can ya do when playing around with eldritch magics?
Since these are the elements of harmony, and therefore closely linked to emotion, I suspect strong feelings heavily influence their effects. Since Celestia was having a bit of a mental maelstrom of anger, sorrow and anxiety over the whole incident, she obviously had rather poor command of the elements. Being the wise sort, she figured that if she were to try again herself once the thousand years were up, the end result would be the same or worse, since fighting her dear sister once more would dredge up the same old storm.
So, in a show of precision planning and a hell of a gambit, she decided to divorce herself from the elements. Of course, she still needed someone else to use them to purify Luna. Giving command of them all to a single being would be too risky (what with folks having this nasty tendency to go mad with power) so she instead opted to spread the power around. Of course, keeping the elements as magic artifacts would have undermined that, since one could just steal all of them. She came up with a rather brilliant little solution: imbue the very essence of the elements into their guardians. Not only did this ensure that they’d be harder to gather up, it would also make it impossible to use them unless the guardians were all willing.
Of course, no-one willingly gives away ALL of their power. That’s where the little amulets come in. The elements require them to act as the focal point of their energies, so they cannot be unleashed without them. And Celestia keeps them in her vault. Pretty clever, no?
So how does that whole “graying-out” shebang factor into this? I suspect there’s a little failsafe in place. The elements can’t be used unless all the guardians are present, willing and free of mind-altering effects. Since Discord’s trickery was decidedly mind-altering, I suspect the elements would have been useless even if Dash was present in the initial attempt. That’s purely speculation, though, but I think it’s fairly likely.