1: Spiral zetsu was created from the gedo mazou. He has the ability to wrap himself around humans and fight for them.
2: Kabuto's backstory was explained during itachi and sasuke vs kabuto.
3: Yamato's backstory was explained early on in the manga.
No I didn't mean that. Those 3 characters were seemed to be completely forgotten after some point but yet they recently have come back to the plot by the looks of it. First, Spiral Zetsu, after around 60 chapters, some people thought that they wouldn't see him again, the person who is inside Spiral who must be Yamato (remember what Zetsu said to madara? he said that the "base" was suprisingly good, no wonder Spiral seems so much more powerful now), and also Kabuto whom everyone thought that would stick in Izanami for an eternity and yet he's back (last panel, that leg).
And in any case, even by the way you misunderstoo what I said, your arguments are still invalid because you what you prove is that it took a very long time before we the backstory of those 3 you mentioned, hundrends of chapters since their first appearance.