I remember them saying the leaf was involved, if they weren't, what possible reason could Kabuto have to hate the leaf and want it destroyed? You're ignoring huge motivations for the characters, just because you're the only person who doesn't remember what was said.
Hidan and Kakuzu are minor characters, they're not important at all except for them being necessary to kill Azuma. They weren't even revealed until their single, small arc that was more about Naruto finishing developing the Rasengan than them actually killing Azuma.
You're focusing entirely too much on the supporting cast. There hasn't been any backstory on Shizune(unless it was in the filler-year), and she's a significantly more important supporting character than Kabuto. They're really unimportant characters overall, not knowing every detail of Kabuto's background isn't going to change the series in the slightest.
You're also ignoring how there is supplemental information on the series that's being published, it's just not being brought over here.