Not sure about the banker being really kind by allowing the no deal to box 23. For me, that's a given. If the mistake's been made, and the honest answer would've been the other one, it follows that the question's asked again and the right answer is given. If the question's not asked again, even if the player requests that, any result from box 23 is null and void, the way I see it. The only way the player's at any advantage is if they ask for the question again once 23's opened and they know the contents. 'And there's...oh, nothing in 23!' - 'Oh, I, er, I meant to say no deal...' - that kind of thing.
But that can't happen prior to the opening...so the re-doing of the question-asking and answering is a matter of course, I think, and nothing outside what's rightly done.