Do you have a theory for Prof. Hobby's return in New York City? His speech, his presence and his unexplainable disappearance make me think I'm missing something.
I reckon Hobby was there in NYC all along. It seems most of civilization has moved inland in response to the rising seas but I see his location there as his kind of private laboratory. My sense is that he has been following David and rigged the Dr. Know to provide the message hinting at his location, basically using David's inherent drive to bring him back to him of his own will. This scene seems to exist, somewhat unnecessarily, to hammer home what is meant by David's actions, in case we hadn't interpreted them for ourselves. I have no explanation for why Hobby would leave David free to roam and eventually suffer the existential crisis that pushes him to jump. And perhaps the fact that a robot could have an existential crisis leading to attempted suicide is rather important.
I'm kind of ambivalent to much of that scene and it, even more than the second ending would probably be the first thing I'd want to cut. In fact, I think it is the first major
flaw in the film. I could picture Joe getting arrested (and not inexplicably coming free again), David getting into the helicopter and flying to NYC following the clues and finding not the lab but the Blue Fairy, skipping that whole scene. And if you need a postscript on the film, you can have David dream his day with his mom, on loop, as he sits frozen under the water. I'm not sure what sense it would make but it wouldn't make less sense than what happens.