I think it was original written this way, with Obi being sarcastic. The changling plot was added in because... well it was just added in. Hey, its the PT. (ever thought what cool things they could have done with a changling? It could have been a whole plot about a fugitive being on the lose. Could have had it take on the form of Obi-Wan or Anakin. I don't get the whole Duku hires Jango the best bounty hunter in the galaxy, who rather than proving his best bounty hunter in the galaxiness hires this changling, who has a little floating robot place some giant centipeds in Padme's bed. Why didn't the changling just make herself look like on of Padme's guards, or better yet, one of her handmaidens then kill her in her sleep.