Thanks
Child and Mr. Underhill for your comments. Sorry that I had missed your earlier query, Child. I am rushed these days. I have only three volumes of HoME. They cost, in paperback, $25 here.
So, the Prologue was written concurrently and finalised after the book was finished. Likely then it became an apparatus like the Appendices to establish the fiction of recorded history and to incorporate material which Tolkien felt would ruin or disrupt the pattern of the narrative. He had so much to juggle!