Ok, so I think the argument has turned from whether or not Celestia is bad in the first place, to what would happen If Celestia was found out to be bad.
The results would be terrifying.
Lets look at it in a "Deathbattle" viewpoint. Five contestants: Celestia, Luna, Twilight, Cadence, and Equestria's pony population itself.
Let’s compare stats then shall we:
Celestia has a very large army, most likely poorly trained. Since we are using Starwars as our reference now, Celestia has the imperial Fleet and storm troopers behind her.
Most of Celestia's guards would probably become cannon fodder for their princess while a few may defect to the citizenry. Unlike the Empire's army Celestia's forces will eventually run out. She can't clone them (actually it is unspecified what magic can actually do) unless she is willing to use the mirror pond. I believe that would lead to a continual degradation of her already nigh useless troops and do absolutely nothing to help her position. And she can't get them from an outside source. The people are already revolting, and drafting them into her army would only give them a way to siphon imperial resources to said rebels. Celestia herself is a powerful pony, wielding Alicorn magic with ease.
While we don't know which side Luna would actually join, I assume her ties to her sister will be the strongest and plan for her much better trained but far less numerous forces (Jabba the Hutt's mercenaries) To join Celestia. Which ever side Luna joins though, I think they will all follow her wholeheartedly. Who else seems to accept them? Luna is also like her sister just a bit less powerful.
Shining Armor, Cadence and the crystal ponies. They would all have the same goal of protecting homelands, but these guards will be wracked with divided loyalties. Different factions will assume they know best how to protect their familiesl. I assume that the guards would at the very least, split into three groups: Those that believe in the necessity of Celestia's continued support, Those that think Celestia has become a safety hazard, And those that hold honor and loyalty to their captain, Shining Armor, who in turn would follow his wife, Princess Cadence.This still leaves us wondering where her loyalties lie. To the princess who gave her the kingdom in the first place, or the Crystal Empire she now rules over? For argument's sake we will say she will do her best for her Empire. This still gives her the same choice as her army. To join Celestia if she thinks going against her will destroy her land, or remove the ally turned enemy who now threatens her kingdom.
The mane six represent the Jedi council. The best in all of Equestria, each able to take on armies singlehandedly, but a loss of a single one is dramatic. They are all across the board. Three scenarios can arise here: One, Twilight continues to believe Celestia is good and follows her and the others follow Twilight in turn. Two, Twilight attempts to join her mentor but the others agree that’s a bad idea and keep her from reaching Celestia. Third, Twilight and her friends realize that Celestia is a threat to harmony and attempt to take her down.
And Equestria’s people. The rebel alliance. A coalition of mismatched vigilantes calling for peace. They are nothing significant, mainly owing victories to outstanding soldiers who picked up slack of lesser ones.
The actual war:
Celestia has a problem.
Apple exporting businesses are getting mad over taxes and prices. After a lot of runaround they start rebellion. (sums up moonlightavenger’s spoiler.)
She begins looking for allies to help silence these farmer/merchants and finds that only her sister is willing to help. Cadence realizes Celestia as a threat and Twilight’s eyes have been opened. Celestia begins sending regiments out across Equestria in waves of freedom trampling hooves. Some are sent to Ponyville to nullify the threat of the Mane 6 traitor scum. Leaks within Celestia’s army help them escape ponyville and Celestia’s trap, however a few of Luna’s guardsmen catch them in the act and manage to eliminate one of the targets and executes the mole for treason. The other five escape and escape to refuge in the Cloud City Crystal Empire, where Cadence accommodates them. Soon Solar Imperial troops reach the crystal empire, led by none other than Celestia herself. She convinces Lando Cadence to turn in the Jedi Mane 6 in return for amnesty and safety of the Crystal lands. With most of the significant enemies out of play, The Solar Empire steamrolls the rebellion and returns life to its previous idyllic state.
Maybe over the years petty underground rebellion affiliates poke their head up and are promptly squashed, or maybe the gryphons do decide to get involved and they decide to help the rebellion, then dragons get involved in an attempt to make a land grab and burn everything then Discord realizes he himself has never in his entire life caused this much chaos and grabs a bag of popcorn and roasts a s’more over the dragon burned remains of Canterlot while watching the fireworks.
in the end, either Celestia wins, or Discord wins. Such a rebellion would be futile.