Fair point, and that git commit will definitely help prevent this. I was told double signing penalization was deactivated, so it's a non-issue for now anyway.
Having said that, there's a user who is currently undergoing the scenario I laid out! @proctologic set himself up as a witness, obtained a substantial amount of votes, and decided he'd rather mine instead. He unvoted himself as a witness, and began to mint PoW. But he's still high up on the witness rankings due to others' votes. An interesting predicament.