Oh cool, someone else interested in Sunset Shimmer. I read it, and have 2 things to say:
(1) During his Youtube review of Equestria Girls, Digibrony/Digibro gave his impression of Sunset. I found it very informative, and also very affirming as it was pretty much how I wrote Sunset, except in action not analysis.
(2) Digibrony was wondering why Sunset, who is supposed to be smart, made the dumb plan of trying to conquer Equestria with just a schoolful of teenagers. Not much of an army. I'll reiterated what I thought Sunset's plan actually was...
Her brainwashed army wasn't as much an army as it was a horde of willing hostages. There was no way Celestia was going to order her soldiers to go and kill an enemy army of mind-controlled children, and Sunset knew it. Her real goal is to force Celestia to an one-on-one duel. Sunset wanted to dethrone Celestia and validate herself as worthy of becoming Princess, by virtue of personal might.
Her fundamental driving goal was never to "conquer Equestria" through mundane war. What would that prove? Her goal was to be personally superior to the Sun Princess, to one-up her. To show everyone, Celestia in particular, that she is born to be a titan among ponies.
This is not because she hates Celestia for no reason. Of course she does like power, and is ambitious and prideful. But the main reason she is so focused on Celestia is because She told her "No, you cannot be a Princess, i.e. I the Sun Princess decree that you are not good enough." That is what she absolutely could not abide by.