I agree with you and Steven that Rory Muir's Salamanca 1812 is a new kind of model for a book on a battle. The division of each segment of the fight into a chapter on what happened and then a later one about uncertainties or conflicts in the evidence shows his maturity as a scholar and his trust of the reader. I would only have one reservation and one question about this technique, which Rory himself may chose to answer.
My reservation would be that the Salamanca 1812 technique of having a whole chapter of historical 'back story' works extremely well for a committed reader but I'm not sure it would for the average member of the public with a general interest in history but little background in the period. This is no criticism, clearly it's a scholarly book, merely a feeling that the technique can only be applied in certain situations. My question would concern some of the other great battles of the age, like Eylau or Austerlitz, where there are fewer reliable memoirs. I really wonder whether the tale of the battle could be written in the way Rory did Salamaca.
As for your more general point about authors being more certain, in the interests of narrative flow, than the history can support, I can see that is a risk. But heavens, it's not a bad problem to be wrestling with after a century of crushingly dull history like Petre !