Thought it was Danzo for the longest, but I feel like he wouldn't be bald
So if this is Shisui, who was likely stronger than Itachi, and he has some small Juubi piece plus White Zetsu/Shin army under his belt, what is stopping him from defeating Team 7? He has S/T Teleportation, Izanagi, and probably all the entailing Uchiha abilities. Him getting defeated would be worse than the way Kishi did Kaguya
I thought so too, but as I write below (you will see) I think that was just used to throw us off because Kishi knows the first thing we will assume is that there is only
1 other person in the entire series to have something similar to that arm and/or features AKA Danzo.
Also, I know you wrote likely but I can't see Shisui being stronger than Itachi. I think Itachi looked up to him and it was stated several times that Shisui was like an Older Brother as well as a Mentor to Itachi, but even Danzo didn't dare ambush/surprise attack Itachi like he did Shisui - even though Danzo definitely planned on it as we see.
It can't be. Danzo took one of them and it was destroyed when he blew himself up fighting Sasuke. He gave the other one to Itachi who then gave it to Sasuke.
Itachi didn't give it to Sasuke - he inserted it within one of his crow's who he then put into Naruto, HOWEVER when Itachi is E/T by Kabuto, he uses the eye's technique on himself and then destroys the crow along with the eye. So none of Shisui's eyes exist AS FAR AS WE KNOW.
It is most likely Orochimaru. He has been behind the scenes of everything INCLUDING and MOST IMPORTANTLY - as in regards to this topic - the research on Danzo. This Hooded-Man has the same arm that Danzo had and Danzo had it because of Oro's experiments and research.