He will look something up and run with it and frequently miss the sublety of history and language in the mix
Don't we all. The subtle nature of history and the language is a relative thing. Backing yourself into Bill's supposed faults at a slightly slower speed than he does is not that big a deal. It's going forward that might get you moving in a more fruitful direction towards such an unreachable goal.
Reading Flaubert, for example, without knowing that as a young lad he used to watch cadavers being dissected, some fresh from the guillotine, and his father throw extraneous bits to dogs waiting outside the window, might well lead to not quite understanding his literary foibles and his general view of human nature.
We are habituated into thinking we have a good grasp of everything that our beloved Media reports to us. And it's broad-brush stuff. And we get used to it and start broad-brushing everything ourselves in our worship of our betters.
When, or if, the Duchess's child become a monarch there will be biographies written about him or her.
Would a biographer include this regrettable incident?