Manwe's speech is from the Silmarillion, as published for what it's worth. I have not checked any of Tolkien's post-LoTR drafts to see if this speech appears.
This is, of course, the difficulty in attempting to analyse this and many other issues in any of Tolkien's writings beyond the Hobbit and LoTR. We can drive ourselves crazy arguing about whether CT's Silmarillion is in any way "authoratative', and what JRRT intended and when.
Letter 153 dates to 1954 and discusses Elladan and Elrohir delaying their choice and attributes the choice to all of the Half-Elven. This Letter also states that Elros' choice was irrevocable and binding upon his descendants, by the way. Letter 211, from 1958, states that the names Elladan and Elrohir both signify "Elf+Man".
From these references, we can make predictions about what Tolkien may ultimately have decided, but even then there is no certainty. Until a final version hits the bookshelves, he could change his mind, as he often did. The problem is he never published the Silmarillion or any of the related works found in Unfinished Tales and HoME. And the last thing I want to do now is try to decide what is "canon".
