I really don't understand why people think Kingdom Hearts story is unfollowable. It's really not that convoluted of a series. It's very abstract, but if you are actually paying attention to the game then you shouldn't have a problem following it. Birth By Sleep was a fantastic game. If Nomura just kept the same old gameplay of the first 2 games then KH would be such a drole series. The gameplay elements (i.e. combat, story, extra features) of BBS was at the top of KH games. **SPOILER** We find out that Ventus' heart is actually within Sora's awaiting its reconstruction. We also find Aqua fell into the Realm of Darkness protecting Terra, and that Xehanort abducted Terra's body to produce Xemnas, who is the leader of the Organization 13 **. KH 358/2 Days is a little bit more boring than the other games, the gameplay is very redundant, but it helps explain a ton of questions from KH2 like who the hell Roxas is and why he was in the organization AND we find out about Xion who happens to also be apart of Sora. I mean the story isn't that complex it just involves complex and abstract ideas (like dreams) to relay relatively simple themes.
TL;DR - KH isn't convoluted, pay attention to the games.