"they both come from stories that had never before been adapted to the screen." Really, what story by Tolkien was Rings of Power "adapted" from? The history of the Second Age is outlined in the back of Return of the King which the writers have ignored, instead concocting incoherent fan fiction.

There's no point in trying to reconcile ROP with Tolkien's actual writing or time-line. The writers didn't need to have rights to the Silmarillion when the history of the Second Age is outlined in the back quarter of The Return of the King; that's a feeble excuse for the incoherent fan fiction they've conncocted using random names from Tolkien's legendarium. ROP is neither Tolkien nor good story telling.

The Council of Elrond in the book is 50 pages long; a great deal was discussed but that would have bogged down the film, which had to keep it's pace going. Reducing 63 hours of reading time to 12 hours of screen time meant that a great deal of the original narrative had to be left out.

Reducing 63 hours of reading the books aloud to a little over 12 hours of film time, and having the story make sense to those who've never read them left out a huge amount of the narrative.

Adding a couple of minutes to show that Denethor had driven himself made by trying to use the palantir of Minas Tirith would have explained him going nuts during the siege.

Making a film out of LOTR meant reducing 63 hours of the audio version to 12 hours of screen time, while the compulsion to make a trilogy out of a one-volume children's book meant bloating the story with garbage, pointless video game action and second-rate CGI.

They had fulfilled their ancient promise to aid in the war against Sauron and were not living entities who could decide to continue. In the original narrative they eliminated the pirate army of the Corsairs, removing them as further reinforcements for Sauron's army seiging Minas Tirith, but didn't show up at the battle, unlike the film version that employed them as the "scrubbing bubbles" army.