This game is the remake of Kingdom Hearts coded, a PC game that was released in Japan only between the years of 2008-2010 in different segments which were the levels used in this game (notably the last segment was released after Kingdom Hearts: Birth by Sleep (2010) as the ending used after completing it contained spoilers related to the aforementioned game).
The opening implies Naminé didn't start messing with Sora's memory until he entered Castle Oblivion.
The opening has images of Terra, Aqua, Ventus, Roxas, Axel and Xion appearing as glitches. Makes sense, they are not part of Sora's journey in the main games.
Maleficent realizes that the Datascape is similar to the Book of Prophecies. Setting up her scheme in KH3D