The preview has me convinced that he's Garth. Cumberbatch was carrying himself with the regal madness of a great man who has fallen. To me that screams the Hero of Axanar more than Mitchell.
The only reason I've backed off Garthism is that Cumby is just too young to play the character. He's the same age as Pine, and Garth should be Pike's contemporary.
The possession angle seems right. Mitchell is still aaaalllivvve and took over this poor Harrison chump. Thus Cumby is playing two roles. This gives Abrams a golden opprotunity to frak with us on a technicality. I wouldn't be entirely surprised if that was the reason behind this plot twist.