You've done everything I would have done. You've offered alternatives, you've stated your concerns which are the same as anyone else would have, and they've ignored you. Honestly, it sounds like they are trying to bully you into complacency.
If they can't understand that you, the sound expert, can't get what they want but have alternatives that would work somewhat (get them clean tc stamped audio at least) and refuse to listen to the dude they hired because you know more about that job than they do, then it might be time to walk away.