So. bets on who Mr. Rabbit really is?
Barring some wacky "He's much younger then he'd have to be to sire Jessica and Arby" science thing, he presumably can't be anyone in the group. That leaves:
The Assistant: Seems too obvious, but the likely candidate.
Milken: She's the one who knew the story about how Rabbit killed everyone and/or made it up to scare people. No reason "he" has to be a man.
Donaldson: Could be, he has the scientific expertise and is hiding in plain sight.
Geoff: Seems to be following the Assistant's orders.