I've thought about that anomaly many times. My own take on it has been that Roberts, knowing full well the unconstitutional abomination that he was taking part in, simply choked on the recitation of the oath. He simply couldn't bring himself to carry through in front of the entire country. This of course led to a second "swearing in" behind closed doors.
By design? Possibly. Literally nothing would surprise me about the activities, both overt and covert, active and passive, that have worked in concert to bring about and maintain this gargantuan fraud.
That's one of the best theories I've heard and I'd really like to think you're correct, as I have high respect for Justice Roberts (definitely one of the things that W did right). However, like you, nothing surprises me any longer.