[Mithadan]

Tolkien's correspondence with Unwin and Waldman was in early 1950. Deducting the roughly 10 years during which he wrote LoTR and worked little, if at all, on The Silmarillion, by the time these letters were drafted, Tolkien had been working on the latter for somewhat over twenty years. Cottage of Lost Play was begun around 1917. When the letters were drafted, The Silmarillion was a not entirely coherent narrative of likely less that 100,000 words. While Tolkien may have "counted" incorrectly, I have no doubt that he contemplated a significant expansion of the text in the order of three to six times its existing length. Whether he would have maintained a base text in the form of the almost historical narrative that he had created with added "appendices" of expanded versions of the three great tales, or whether he hoped to expand the text in its entirety cannot be known with certainty. We can glimpse what he intended in the versions of Hurin/Turin and Tour/Gondolin found in Unfinished Tales. That he never actually put his vision down on paper in its entirety is tragic.

I do not think he would have included The Lay of Leithian (except perhaps as snippets) in the Silmarillion. He had abandoned any realistic hope that it might be published even before Unwin "rejected" it. His correspondence predates the Athrabeth but I am willing to believe that it might have ended up in an appendix. I believe Akallabeth would have been included, but not Tal Elmar.

William the wordcount for LoTR that I mentioned came from an on-line source, and I do not know how reliable it is. It is consistent with estimates I have seen that a page of text in a book averages between 250 and 300 words. As I have no intent of sitting down and counting the words myself, I defer to you. But either way, 300,000 or 600,000, considering that it took Tolkien ten plus years to write LoTR, we can assume that it would have taken at least that long to rewrite The Silmarillion to satisfy Tolkien's exacting standards. However, his correspondence with both Unwin and Waldman seems to suggest he wanted both published simultaneously. I cannot conceive that a publisher would sit on LoTR and wait for The Silmarillion.