It's not Shanks, and it doesn't have to be an identical twin, just needs to be a brother or an unidentical twin. This other dude is definitely Shanks relative and he's going to be a major player in the final war, or he wouldn't have been shown in that montage of all the finalists.
If there is no twin, then the person talking to the Gorosei is Shanks lol. Its literally his face.
There isnt a good way of spinning that theory to make sense. Nothing specifies Shanks has a brother (he might, not the point im making), but that doesnt detract from the fact that:
- Garling isnt blamed or punished for his Son being a Yonko, having ties to Luffy or Roger, or being ever closer to One Piece.
- Shanks has met Gorosei (again, if not a twin it has to be Shanks; and a twin doesnt even make sense because why would he have needed to hid himself?)
- Shanks had the respect of Sengoku to stop a war.
And the list goes on.