Looking at the facts from the manga it could be either Black Zetsu controlling Obito or...actually I can't think of anyone else who might be in there. I doubt it was ever Kabuto under there since he didn't like being exposed. Hence why he chose Edo Tensei as his specialty.
Nah,it is definitely not Obito he is in no shape to move now let alone fighting whole SA at the same time using high level wood style and he is currently being controlled by black Zetsu and there is noway in hell for him to escape from Minato and Kakashi.The Zetsu that is fighting SA is spiral Zetsu but the Zetsu that is controling Obito is black Zetsu they are completly different from eachother so it is obviously not Obito
IMO he is Yamato or Nawaki.
As far as who might visit Sasuke...I am leaning towards Madara or Kabuto. Madara was in his vicinity anyway and well Kabuto seems a bit unlikely given the distance it would have taken him to travel. It could be Onoki (Tsuchikage) since he was barefoot as well. He seems to be one to share a similar experience with Sasuke. I mean he solicited the Akatsuki and was quite stubborn until Gaara gave him that bit about, "...when did you forsake yourselves?..."
Well,Madara said that everything he need is his left eye now he is going after Obito.No need to mention
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so it is obviously not Madara nr
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and Onoki is currently fighting S.Zetsu and almost has no chakra left on top of that he can't do anything to save Sasuke at this point.Here only person that makes some sense is Kabuto,he is one of the few people that can save Sasuke and his legs are similar to that of the person that is approaching Sasuke and Kabuto accepting himself and saving Sasuke would fulfill Itachi's last wish.