OK, to start with, I am not throwing out my previously suggested idea that Jay Garrick is Zoom. (Maybe there's a funky explanation coming. BTW, Jay's been the one doing all the stuff on the speed cannon.) ;)
Anyway, here's another idea about Zoom: He's Jay Garrick's identical twin brother!
Earth-2 Mrs. Garrick had twin sons. Both eventually became Speedsters, but one got separated and then eventually got adopted by the Zolomon family
Earth-1 Mrs. Garrick had twin sons. One got separated and then eventually got adopted by the Zolomon family. The other son got kidnapped by Zoom before E-2 Jay Garrick got to E-1. Somehow, Zoom already knew the location of E-1 and how to get there.
E-1 Jay Garrick: guy in the mask
E-1 Hunter Zolomon: guy on the bench
E-2 Jay Garrick: Flash
E-2 Hunter Zolomon: Zoom
I know there's gotta be a hole in my theory somewhere, but, ih, whatever.
EDIT: What if Jay Garrick is E-2 Jay Garrick, Zoom is E-1 Jay Garrick, and the MITIM is E-3 Jay Garrick!?