I'm not sure exactly when it was released. You might want to check the official publisher's website or some reliable book news sources for the accurate date.
The back story of KH is quite complex and involves a series of events and characters. It all started with a mysterious incident that set the stage for everything that followed.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3) or FanFiction.net. Just search for 'Kingdom Hearts Xemnas Saix' on their search bars, and you'll likely find a variety of fanfictions involving these characters.
Since I have no prior knowledge about what 'kh 358 2' represents, it's impossible to tell the full story. It could be an identifier for a local event, a private initiative, or a specialized item. If you can provide more background, it would be possible to give a proper account.
A 'kh crossover fanfic' is a fan - created work that combines the Kingdom Hearts (KH) universe with elements from other franchises. It could involve characters from KH interacting with characters from, say, Final Fantasy or Disney in new and unique storylines.
One possible fanfiction could be about Roxas and Xion exploring the new worlds in KH3 together. They might face new enemies and challenges while also reminiscing about their past. Their bond could grow stronger as they help each other through difficult situations.
In most cases, the KH light novels are considered canon. They often expand on the existing lore and characters in a way that fits within the established framework of the series.
Xemnas in Kingdom Hearts 358/2 Days is a complex character. He's a key part of the Organization XIII. His story involves his manipulations within the organization. He has his own motives which often conflict with the other members. He's constantly scheming to achieve his goals related to Kingdom Hearts, using Roxas and the other members to further his plans without them fully realizing it.
One key element is his leadership in Organization XIII. He guides the actions of Roxas and others. Another is his pursuit of Kingdom Hearts, which drives most of his decisions. Also, his relationship with the other members of the organization, especially how he manipulates them for his own goals.