I believe Catlady was referring to the only credit worth having: credit with the reader of this book. At the end the reader is left with the impression that Grant has kindly bestowed his knowledge on the American, wanting nothing in return. So in the fictional world the credit may seem to be with the American, but in the real world the reader knows (or seems to know) which character deserves the credit.
And I'm with Catlady on this, at least to some extent. It was Carradine that acted as the true historian, referencing the work of past historians. Grant is there to act as the reader's stand-in, to make all the wrong assumptions only to have them refuted by the next layer of information revealed. Oh, and he's also there to make disparaging remarks about historians and their lack of ability to see what makes sense. If only that had been tongue-in-cheek, this might have been amusing: the real conclusion to this should have been Grant recognising how ironic it was that he spent a book criticising the ignorance of those whose work he relied on to expose his own ignorance.
But we face the same problem with the painting he asks everyone to judge, a painting at least two removes from the original painting, itself an artists rendering of the original visage. Why is Grant's phrenological belief in faces never exposed for the farce this painting of a painting of a painting exposes it to be? Sure, I really like that painting - compare it to the rat-like features of Henry VII
shown on Wikipedia at the moment - but to try and read whether that face was capable of dispatching two young boys is ludicrous in the extreme. Only the doctor got it right when he said: "I suppose villainy, like beauty, is in the eye of the beholder."
I actually like the idea of using the painting as a way into the story, but it seems a big mistake to have an experienced detective portrayed as believing such nonsense.