I do not agree that the matter of internal or external is that simple as Galin put it. Let's take up the example of Amroth again. At first view Galin is right. Since Tolkiens is drafting diffrent version at diffrent times this seems to be claer case of external uncertainty. And we all can be sure that Amroth and Galadriel had never any doubt about the matter (alltough Celeborn might have
).
But that is not the end of the story. A secondary reality is formed in the mind of the reader while he reads and reflects about it. And Tolkien left diffrent versions. So in the mind of the reader uncertainty is created. Obviously the degree of uncertainty differs individualy. So you, Galin, might have found a clear line how to decied which version of Amroth family is true for your secondary reality of Middle-Earth. But other poeple might find even clearly reported events in Middle-Earth so unbeliviable that their secondary reality of Middle-Earth contains uncertainty were you would never have expacted them.
Better categories then internal and external in the case of uncertainties might be intended and unintended by the author. Thus the case of the Elessar or Tom Bombadil might be intended and Amroth might be unintended. But we come to a grey area when we look at the question were the Orks come from. The account in the Silmarillion is uncertain by intention, but we learn by reading further that the author himself didn't know the 'true' answer, so the uncertainty in the text was probably forces upon him by his own uncertianty.
Respectfuly
Findegil