It also bugs me that they explicitly say that John Constantine fixed Sara, and then just don't mention it again. Did he raid the liquor cabinet? Did he leave the lair smelling like smoke? Why don't they call him!
Also Felicity is a tech genius with a tech genius assistant and is the CEO of a billion dollar tech company…

My theory is Zoom is Eddie Thawne. Either Earth-2 Eddie, or Earth-1 Eddie who got somehow revived and supped up after his body goes through the breach.