Probably not. G1 and G4 of My Little Pony are largely separate entities and Hasbro no longer has the rights to most of the G1 characters.
Yes. The Villain in Twilight's Kingdom has been revealed to be named Tirek.
It is not known. The origin of Discord is not discussed.
Not likely. Such a subject matter is probably too dark for My Little Pony.
Yes, Cadance will appear in the My Little Pony Friendship is magic episode Twilight's kingdom.
Yes, the shining armor will appear in the My Little Pony Friendship is Magic episode twilight's kingdom.
No, it has never been announced or in any way suggessted that Justin Bieber will appear on the television show My Little Pony: Friendship is Magic.
No. She does not appear.
Probably. When he flew away after the library was destroyed, it is unlikely that Twilight was just letting him go. Most likely he was just flying off to a safe place, away from Tirek.
No. Justin Bieber was never cast for the television show My Little Pony: Friendship is Magic.
Yes, Princess Celestia, Luna, and Cadence will get their magic back since the main 6 defeated Tirek and have been rainbowfied to help Equestria receive their magic back. If you want to watch it you can on Youtube or Dailymotion
My Little Pony: Friendship is Magic
My Little Pony: Friendship is Magic premiered on October 10, 2010.