So, I'm finally caught up. And I'm confused.
As best I can figure / guess / wildly theorise, Zoom is Jay Garrick from the future. In the original timeline, Jay Garrick helped The Flash close all of the breaches, but became addicted to Velocity 9 in the process. Upon returning to Earth 2, his addiction to Velocity 9 eventually drove him insane. Velocity 9 potentially stopped having a major impact on him and he became obsessed with stealing speed from others, leading him to travel back in time to when he knew there were portals to other worlds with the intention of stealing The Flash's speed.
I think that would explain the reasoning why TPTB wanted to include an episode showing that the Reverse Flash still exists, even though Eddie killed himself. Because they needed to "explain" how Zoom would still exist once Jay Garrick was killed, despite it being a complication for Zoom.
BUT, that doesn't really account for all of Jay Garrick's mysterious absences this season or "Hunter Zolomon" on Earth 1 (since that name indicates that he has a larger role to play than simply being Jay's doppelganger) or the fact that the man in the iron mask really seems as though he's another doppelganger of Jay based on hair colour, etc.
I really hope that TPTB have a convincing explanation for everything. I'm NOT a fan of Jay Garrick "playing" Team Flash, especially Barry and Caitlin for the season to date. Not only is it a repeat of the storyline from season 1, but it is also disingenuous to the comic book character of Jay Garrick and to Barry and Caitlin's characters.