When the cows come home
Yes. This has recently been announced.
Probably not. It has been said by the production team of Equestria Girls that Equestria Girls was meant to be a standalone, and not be connected to the TV show My Little Pony: Friendship is Magic.
The Equestria Games arc of My Little Pony seems somewhat similar to the Grand Galloping Gala arc of season 1. So it is logical to conclude that it will be right before or part of the season finale.
yes
No. It has been stated that season 6 will focus on exploring Equestria.
Season 1, Episode 1: Friendship is Magic
No. It is the last episode of season 3, but there will be a season 4.
It is not known. The only information known about season 5 is that it will have 26 episodes.
This episode has already been released.
Probably. Since the box was introduced in the pilot and the Mane 6 finding their keys has been a story arc for the season, it will likely be concluded as part of what appears to be a two-part finale, as episode 24 will involve the Equestria Games.
Photo Finish is in season 1 episode 20 "Green isn't Your Color"
No. It was the last episode of season 3, but season 4 started up on Nov. 23rd, 2013