Here's my thought process...
I think originally they were going to have Zoom be an evil version of Jay, and Jay would be the Man in the Iron Mask.
Now, I think Zoom/Zolomon is going to be a totally separate entity from Jay Garrick - the REAL Jay, that is. I still think Jay is the Man in the Iron Mask, but I'm also open to the possibility that the writers may change their minds and it may be someone else, such as Eddie Thawne. I also think if the Man in the Iron Mask is Jay, it will be a different actor portraying him.