My only guess is, the intent with the four arms was going to be to have it molting into a Queen. But who knows for sure.
As for the Derelict being made out of "human DNA" by the Aliens, it's silly/out there but also, it's the element from Blomkamp's overall concept that I found to be the most interesting. I'd be curious to learn more about that piece of the puzzle. I'm sure it wouldn't have been particularly good, but it's way more interesting/wild a concept than I would have expected from a movie that otherwise seems to have had the aspiration to be an adaptation of the Kenner Aliens stuff, more or less.