What then If/when Nightmare Moon rules Equestria. Become a cruel dictator, everlasting night, change in leadership with a stronger military force, better health care? To be continued . . . .

Finally

Looking forward to seeing the new regime. Hopefully no shitty Deus Ex Machina ends it within 3 chapters.

So...how do horn injuries work in this universe?

6202422 Breaking a unicorn's horn does not disable their ability to use magic; it would be the equivalent of a human losing a fingernail and expecting them to be unable to grab things. Additionally, breaking any section of a unicorn's horn above the mid-point of their length does not cause pain; it is near the base of the horn which holds the nerves.

A unicorn's horn grows back incredibly fast - an expected span of two to three weeks when losing more than half - and even surgically removing the entire horn does nothing to stop it from regrowing from scratch. It is the only appendage of all the races that can grow back naturally - an equivalent would be a human regrowing an entire limb.

Even so, this does not change the fact that attacking the horn is a good strategy when facing a unicorn.

6202740
It was more of a shock factor for the guards, wasn't it?

6203955 The shock factor was a side effect of what Nightmare Moon truly intended when she broke Celestia's horn.

6203955
It may have the same effect as removing the fingernail, but it feels like having half of your head removed.

Nightmare Moon took a big risk because had Celestia known the workings behind the spell, the princess would have simply moved to the side; the spell has no tracking capabilities whatsoever.

I thought Celestia DID move?

7562295 She only moved backwards; I should have made it a bit more clearer.