I like the idea of Hunter Zolomon, but his time manipulation ability doesn't tally with Zoom's obsession with the Speedforce. As to Edward Clarriss, well there is logic in that theory but consider that good writing means setting up your props and players early on and the unmentioned Clarriss falls into Johnny Chambers categoray - I do not believe the writers would pull in such unseen and previously unmentioned characters at this stage.
The #1 theory seems to be that the 'Iron Man' is Jay Garrick, while the clues given to this could well turn out to be a deliberate misdirection it makes some sense that Zoom might be tied to using this Jay as a power battery. There are definitely elements of Clarriss' origin and appearance here though I agree. Much more than there is of Hunter Zolomon.
Thinking on it further it does seem to me that whoever Zoom originally was he occupied the same space Barry did on Earth-2 and took his place in the Particle Accelerator accident. Barry is nowhere near as fast as Zoom, or so we are led to believe, but given the fact that Eobard Thawne altered his timeline and ensured a much more hesitant Flash came about, always checking his speed and potential, what if Zoom's speed and power is actually just what Barry was in the original timeline, and could be again when he overcomes his crippling self-limitation?
I agree that the man in the iron mask is the real Jay Garrick. I also agree that Edward Clariss/the Rival is too obscure a character to pull out of nowhere like this. So who is "Jay" really?
My theory: The version of Jay we've seen is actually the Earth-2 version of EVERYMAN. That's why he needs Velocity serum to use the speed force: his speed powers were never drained, he never had any natural ones to begin with. We saw Everyman pretend to be Reverse-Flash last season, I wouldn't be surprised if they pulled the same trick again with this supposed Earth-2 Flash.
They've gone out of their way to show that people who become metahumans on both Earths tend to have the same powers (Cisco, Ronnie, Geomancer), so it's not unlikely that the Hannibal Bates of Earth-2 wound up with the same shapeshifting abilities.