Cymkin's Conspiracy Theories

Heres what I see happened
I don't think celestia is as powerful as we think. For I believe that discord seeing that if they were to imprison him, he would remain that way so long as they were alive and well and connected to the elements. So in a final act of desperation he planted jealousy in luna's mind so that she would one day betray her sister and sever their bonds with the elements. After a lengthy battle with nightmare moon celestia had to make a decision, she could not restore luna's mind because she lacked full control over the elements (they shared control remember) so she did what she could to buy herself time. She quickly sealed nightmare away and knowing full well that she her seal as well as discord's would not hold without power over the elements she set her plan in motion. She would wait for one of great magical talent who could retake control of the elements, she would the aid that individual on their path towards this destiny. She was preparing for the day these two would break free from their banishment. The reason I think she was so surprised when discord broke free is that she couldn't really estimate how long his seal would last.
As for sombra, I don't think luna and celestia sealed him I think he did it himself probably in some act of trying to outlive the sisters or trying to recover from mortal wounds he received during battle (notice the form he returns in and that his magic isn't at full power). Im pretty sure he'll return because if you look you'll notice a piece of his horn is flung by the whole crystal explosion thing. But that's just my opinion. Blow your mind did I?