The Secret Ending is straight from the game.
No it isn't... Maybe a small segment may be canon, but that's about it... The keyblades (KK, IK, WtD) are not canon, but show a connection toward Sora, Riku and Kairi from VAT... The first shadow, which looked very similiar to Xehanort changes into MX in the second one, possibly implying MX will become (probablly half) of Xehanort... MX and the DS split from one, possibiliy foreshadowment of their fusing... The secret ending is not taken from the game, but simply reveals secrets from it... Meaning, the "keyblade graveyard" has a 50% chance at being symbolic toward something else...
I think this is the most awesome theory ever.
No offence toward you or Yannis, but this is far from the best theory ever... It's good, but not best...