The "Shanks" who appeared in the room of power to talk to Gorosei about a certain pirate should have been the real Shanks.
Otherwise, Gorosei almost dismissing his request to meet them wouldn't really make sense if this was a Holy Knight.
Unless, that Holy Knight is deserving such treatment, for some reason.
Regarding the summoning circle, this is either a technique that anyone can use, if they know how to perform it or it is a power of one individual and all others are borrowing it...
What I wonder about is whether this can be used in the Afterlife to come back alive...
Considering Oda's trend of turning One Piece into a story where nobody stays dead this could really be OP's Edo Tensei.