1)everyone, wouldn't play BBS and then start questioning who that is. a first time player who unlocks the secret ending will know that isn't the first game, so no one in that category would go "who the heck is that!?" the ones who play it as they go, still are aware of her existence, and i doubt someone who does, will know nothing of xion.you said "everyone".
not all fans of these games looks up info on it and what not. some people just play it as they come.
The keyword is "full".
And there isn't really any way to misconstrue what the "KH plot is". It's the plot of all the games. That's the full KH plot. The full story. And every game in this series contributes to that full storyline, which means, if you're looking to understand the "full" KH Plot, every game is a vital one.
as for keyworrd full, i doubt the opening post meant literally "full" otherwise, it would've been a stupid question