As to the behemoths being controlled... in the scene with the hospital, when that horde and its leader returned, they saw several behemoths in the crowd, following everyone else.
In that same scene, Kalani and Riley enter that generator room filled with blood and gore, as well as needles and surgeon's tools it seemed. The zombie horde dragged back a bunch of living humans with them. My theory is that, instead of a hive theory these zombies are being made in the process of the transformation. This would not be taking dead flesh and reanimating it, more like... sculpting a form, perhaps somehow altering the dna by adding various different substances or sticking on other parts. If you took several people's hands and stuck them all on one person, they would all still be alive, and depending on how the virus works it could fuse them together so that its one creature. For jumpers just those muscles are harvested and implanted into the leg, taking extra skin to cover up the muscle. All this would keep any sort of reanimation from happening, and it would explain that room with the smart one (I think it was Ink, just can't remember at this moment).