In Unfinished Tales, it is stated that the Akallabêth (as published in The Silmarillion) was written by Elendil after the Exiles of Númenor settled in Middle-earth.
From an in-universe perspective, how reliable is this work as a primary source for the history of Númenor and its downfall? Assuming that Elendil's main sources were his own experiences and his fellow Faithful survivors.
Some points of debate are as follows:
1) The fate of Ar-Pharazôn and his army: Given that setting foot on Aman is a one-way trip with no return, how could Elendil have known that they were buried for eternity in the Caves of the Forgotten after making landfall there?
2) The death of Tar-Míriel: How could any survivor of the Downfall have known of her death by drowning as she attempted to ascend the Meneltarma?
3) Sauron's glee: How could any Faithful survivor of the Downfall have known of Sauron's laughter during his final moments in the Temple of Morgoth, just before the end?