I am not quite sure what you mean by "negate" in the above paragraph. Normally if a man says one thing and then sometime later says "No. I've changed my mind," and says something quite the opposite, then that constitutes "negation." But I digress ...

I think the philosphical and theological reasons that Tolkien gives for rejecting the Elvish origin of Orcs is sound. He also considers and rejects the alternatives of (1) Orcs being seperately created by Morgoth (like Aulë with the Dwarves), and (2) Orcs being just intelligent animals, and not moral beings. He eventually decides on a Mannish origin for Orcs.

: A little ambiguity is often more entertaining than cast iron certainty.

There is still plenty of that. As a Mannish origin for Orcs must push the Awakening of Men well back before the rising of the Sun (contrary to The Silamarillion). I have my own thoughts on how Tolkien might have avoided this conundrum, but it is a bit complicated to give here.

We also have an apparent contradiction on the question of Orkish lifespans. If they were of Elvish origin then they would be expected to be immortal. But Tolkien concludes that actually Orcs were shortlived compared even to the Edain.

However Tolkien avoids this last problem beautifully: "This last point [Orkish mortality] was not well understood in the Elder Days. For Morgoth had many servants, the oldest and most potent of whom were immortal ... the Maia; and these evil spirits like their Master could take on visible form. Those whose business it was to direct the Orcs often took Orkish shapes, though they were greater and more terrible. Thus it was that the histories speak of Great Orcs or Orc-captains who were not slain, and who reappeared in battle through years far longer than the span of the lives of Men."

Nothing like a bit of entertaining ambiguity to get the great man thinking!