I'm gonna jump back to the whole "face-stealing" thing ...

it depends on three things, whether he'd have to kill someone to "steal their face". Number one: is orochimaru physically taking the actual face of the person (like to wear it as a mask). If not, then there's not much reason to kill them. Number two: is orochimaru able to do any sort of reasonable transformation? If so, then there's not much of a reason to kill someone, just to do transformations. Number three, being a high level ninja with huge amounts of techniques, is it inconceivable to be able to physically manipulate your own appearance (without doing a pure transformation - molding your face just like you mold your chakra)? If not, then again no reason to do so.

I think kazekage died because he's a visible guy and not an idiot. He had to die in order for orochimaru to have some sort of cover - if oro were still in his place and attending the exams and kazekage was around too, then it is possible that both of them would be in the same place at the same time, making it unfeasible for orochimaru to use the disguise to get as close to hokage as he did.

What better way to enable yourself to replace someone completely than to make them absent in the first place?

As far as itachi and the whole killing uchiha thing, I don't see him even trying to claim he didn't. If he ever said "nope, wasn't me, you're wrong" then maybe it'd be in doubt. But hell, he's in the same organization as orochimaru and shark guy are in, and neither of them seems to be a reasonably sane character either. Why should he be an innocent victim of a conspiracy? Why not make him just a complete dick?