Imagine a book written from the perspective of Robin Hood. Throughout the whole book, we have a first person perspective in which the protagonist describes robbing the rich and giving to the poor. He really believes he is doing good work and that the people he is stealing from have wrongly acquired their wealth. They are universally depicted as evil, and he is giving what he steals to an oppressed class of forest / mountain dwelling people that live beyond the rule of the crown. Enter chapter 25 from the view of the Sheriff of Nottingham. It turns out all this time, Robin has been stealing mostly from yeomen, foreign traders, and even some well off villeins. He has not once robbed the nobility, though the nobility's towns have been suffering from decreased trade due to bandit activity, and many more regular people are going hungry because of it. Chapter 25, two-thirds of the way through the book, is the first and only chapter written from another character's perspective. In it, we learn the the Sheriff has a cunning new plan to take down Robin Hood. We also learn that Robin is so beaten up and poor that he mistakes yeomen for knights, and that the people he is giving the stolen wealth to rejected the crown a hundred years ago and went into self imposed exile, refusing to pay taxes. In Chapter 26, we return to Robin who doesn't know about the Sheriff's cunning new plan. Robin remains unreliable, but now the reader knows for sure, rather than simply suspecting. Is this a fair format? Is it cheating? Where has it been done before?
I've seen the tactic employed, though not so fleetingly as to be in just a couple of chapters. Is all the rest a single narrator? I think it may protrude if so and be a little obvious if the flow's broken by these asides. A device for the sake of getting opposing views across. One example of its use was in Ghostwritten by David Mitchell but it formed the basis, the structure, of the whole book in that case.
You should read "The Rules of Attraction" by Bret Easton Ellis. It is the personification of that trope..
I would definitely consider the tactic cheap. A reader should have reasonable expectations. It would work better if there were hints along the way suggesting that something isn't as depicted. As well, it might be best to have a slice or two of the Sheriff, along the way, even if they do not expose the truth about Robin being not as depicted. The sum would amount to the reader guessing, and then having some of those suspicions verified in the later chapter. An unreliable narrator does not mean that the reader can't decipher the truth. It does not mean that we trick the reader. While some of what you propose is fine, the degree to which you wait so long to do a full 180 is kind of cheeky, in my opinion. It's like a mystery novel wherein the reader has no chance of knowing who did it, and then once the reveal happens they are made to understand that it wasn't even remotely possible to guess, all along.
My thinking is that, at least in third person limited, all points of view should have their own innacurracies. The Sheriff in your example seems very objective, but he himself will probably have his own biases. This is a good concept, but executing it well requires complexity and ambiguity I think.
