d20 Pony begins at the end of the summer following the Ending of the End. Canterlot Castle has since been rebuilt, and Princess Celestia and Princess Luna are still officially rulers of Equestria.

The seasons 1 to 9 of Friendship is Magic (up to the 25th episode of the 9th season), the movie, and the specials (Best Gift Ever, Rainbow Roadtrip) are generally canon to d20 Pony.

Spinoffs (such as Equestria Girls, Pony Life, and the comics) and other generations (past and future) are not.

Not officially. The story and artwork both intentionally leave any such details to the imagination.