This is the way I'm leaning as well. Arnold was Ford's first robot he created all by himself. It was sort of his companion and best friend, and in being so it learned how much Ford missed his past family life, so created the family replica for him. They created all the robots together. But at some point Arnold becomes disillusioned with his role in the world and that of all the robots he has created. Cannot bear that he has created all these other beings like himself which have to endure these endless horrors. Being the same as them, he knows they are not just robots. He tries to convince Ford, but Ford refuses to see the humanity in them.
-It was emphasized by Jimmy's friend that nobody had any record of Arnold's life prior to the park.
-We never hear Arnold's last name. I wouldn't be surprised if we hear Arnold was just an acronym for something... Autonomous Robotic Neuromorphic...
-Would be pretty easy for a robot to disappear for 30 years, or "kill himself" while really uploading himself to another host he has created.