"Starswirl invented time travel magic", Starswirl when he is brought back with the other heroes in the present doesn't seem to know about this, so this might be a past version of him before inventing that magic.

Discord in season... 4? The one with Tirek? Seemed like he was told by Celestia or someone that he would need to become friends with Twilight to beat Tirek, though he didn't have all that information.

Remember the blue flu episode?

It was strange how he just shows up out of nowhere and really, really wants to spend time with Twilight, give her a half-assed friendship necklace that "proves" their friendship, and then he fucks off (once he gets better).

Makes a lot more sense when he realizes the necklace he got from Tirek is the one he was meant to give Twilight all along, and it's a true symbol of friendship. Him saying he understands now goes a little deeper when looked at this way, he's had an entire story arc we only saw parts of.

Remember how the box was important and they needed it to beat Tirek and create the map in the new tree castle thing? How did they open the box? They had to check Discords notes which pointed out to them that these items were all keys for the box. What items? The ones they gathered throughout the season and wrote about in the book with the notes. What book? The book Twilight found in the castle of the two sisters hidden room. How did they find the hidden room? Angel Bunny startled Spike and he fell over in the chair which opened a secret compartment. Why was Angel there, and Twilight? Angel was there because Rarity and Fluttershy fell in a trap he accidentally activated and he found his way to the library where Twilight was. Twilight was in the library because Celestia mentioned an important book was there and she would find it, Fluttershy was there because Rarity had been tipped off by someone in Canterlot that there were rare tapestries there and she just had to go see. Celestia, of course, lives in Canterlot. On an unrelated note, Pinkie followed AJ and Rainbow Dash after she stopped ringing the school bell for the day, meaning the only ponies who went to the castle by choice were Rainbow and AJ, the others besides Pinkie Pie went there because they'd been told to by someone else, and it just so happened to align with their competition in a spooky castle.

Basically, defeating Tirek required every single thing in this unbroken chain to occur, and then Discord spoiled the ending of the season because he didn't want to learn a friendship lesson.

Once you look at everything else in the series it makes sense that a time traveler like Starswirl could have learned all this, gone back to his time, helped build the castle of the two sisters such that all these secrets and traps are in the right places, same as the secret compartments throughout the original tree library Twilight used to live in, and cause the events of the show to happen.

Remember way back when Granny Smith told the kids in the school about the history of Ponyville? There was no Ponyville when she was a kid, Celestia told them where to build, it just so happened to be near an orchard of zap apple trees, and just so happened to bring in enough money to get the town started. If everything else was planned ahead of time, it makes sense that they were told precisely where to live so that the town would grow. Many times living near the forest has caused problems other cities don't have to deal with, many large monsters come out of the forest throughout the series, it's clearly a terrible location to set up a small town, but it had to be done because the castle of the two sisters was near enough to walk to from there.

Still, I don't think Celestia is a mastermind, she might know what's going to happen, but everything has been laid out by Starswirl, who invented time travel magic in order to return to his own time, who wrote the book which turned Twilight into an alicorn, and likely prepared all the hidden items they would find throughout Ponyville and the castle.