I'll throw my hat in on "poison doesn't make sense" if he had a poison why not take 2 seconds and give it directly to Hidan while he was bound? Simple solution that doesn't put Shikamaru at risk.

I'm not sure why showing Kakuzu use low level water techniques is a screw-up, it makes more sense that someone that old should know lots of techniques regardless of their type.

Either whatever was given to Shikamaru was Kakuzu's blood and he somehow switched it (doesn't seem likely given that the cut was on his cheek, if it had been something covered in clothes it would make sense) or he is about to wipe away some of the blood symbol of Hidan's with a shadow or something. I doubt he gets hit for real.