So did Discord know how to open the box all along?

[Azureth 687]

Considering he ultimately managed to give Twilight the means to find out about the keys are we to assume he knew all along?

[Nature's Spell 526]

I don't think he knew, or even expected, that he'd be the final key. It would seem he at least knew how the mane 5 got their items was important.

[Ribbonfree 2,384]

He definitely knew about the boxes general function and mechanism considering how it was his prodding that got the six to get the other keys activated. But I doubt he expected that he particularly would gift the last key. I mean he definitely seemed confused during the Season Finale, just trying to figure out where he really belonged, constantly swapping sides. No way any of that was planned.

[Pastel 7,630]

Yeah he knew about the keys and how to open the box.

But he didn't expect to be the one who will give the last key to Twilight.

He was very surprised when Tirek betrayed him.

[Thespian 78]

It is hard to say. No one knew Tirek was around still Celestia had the vision and Luna happened to watch it at the same time. So it isnt like Discord has been setting up this 1000 year old ploy just to make Twilight a real princess. More like he is honestly a pretty weird guy and noticed that those certain items in their journal were important. He must have somehow realized they would help with the box and wanted them to open it using these important items. He probably didn't realize the necklace given to him from Tirek who got it from his brother, who most likely got it from either the ponies themselves or made it because of the ponies influence on him, would be a key for Twilight. Honestly if you look at the story overall, it is a little weak. However, the episode was still good for all the emotions and action that happened as ell.