They even laid the foundation for it by having the resident science genius have a seizure (probably emerging Firestorm powers) and will most likely be written out as a means to make room for Wells/Cavanough.
We already know that Stein will be a main character in Legends of Tomorrow, so, yes, he'll only remain on this show through December.
Also a good point on why Zoom has not crossed over himself though how could he know about Reverse Flash or Jay Garrick?
Zoom has crossed over. We saw him bring Sand Demon through the portal to our world. He's evidently commuting between universes to bring the bad guys across. Though why he doesn't take on Barry himself is unclear.
And he knows about Jay because he's been Jay's archnemesis on Earth-2 for the past two years.