I've read through your argument and neither of you manages to convince me. If you ask me,
Lal is imposing too much importance on Ungoliant (considering how small a part she played in the actual legends) while
Legate is seeking to label her as an Ainu and neither seems right to me.
So, there has to be some other explanation. Unfortunately, I don't have it.

However, I got this intriguing idea that has totally no support from the books. Could Ungoliant have been born of the Music of Ainur? Of Melkor's discord, or of something more planned? Or of the Dark that followed the Music?
~*~
Actually, now that I was eyeing my Finnish copy of the Sil, the translation suggests a new answer. The English passage goes:
Quote:
The Eldar knew not whence she came; but some have said that in ages long before she descended from the darkness that lies about Arda
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and the Finnish translator has translated the word "to descend" in the sense "to be born, be a descendant of somebody" (like "a descendant of Kings") not as "to come down". What if the translator got it correct here? What if Ungoliant was (for some unknown reason) born of the darkness that surrounded Arda?
If Ilúvatar was there first and created everything, he created that darkness as well? So Ungoliant would be a creation of his (in a way) but not his child in the same sense as Ainur or the actual Children of Ilúvatar. Actually, my theory sounds
too good at this point.

Please show me the loophole(s).
PS. Sorry for the messy structure and flip-floppiness of this post, but I often write and think the same time, not think first and write then. (Ask anyone who's played ww with me.

) I figured editing it all into a consistent post would be too much trouble, so I left it as it stands.