Everything goes back to the music. I would argue Ungoliant actually IS among the Ainur who took the form of a giant spider in Arda, so effectively a Maia even if never made explicit. During the music of the Ainur, Melkor’s discord brought many Ainur to his side. This is simply one of many. Sauron is just the most noteworthy.
But there is no basis whatsoever that offspring of Maia inherent their parental powers, be it Shelob, literally all the spiders of Mirkwood, or Luthien. We are also dealing with a time difference of several thousand years. The only maia in Middle-earth during the 3rd Age are the Istari, some Balrog who escaped the War of Wrath, and Sauron. (arguably Tom Bombadil) and none of them can change form. NONE! (arguably Tom Bomadil can)
So essentially you are promoting spiders to full maiar status complete with shape-shifting powers, something even Sauron cannot do. (any more)
As an aside, there are numerous versions of Professor Tolkien’s earlier works and while he never settled to finish The Silmarillion to his satisfaction in his life time, it would be useless to argue against the version we got. Many books in the Tolkien Legendarium are decidedly not canonical. I only say this because some things you mentioned are actually spelled out in the text. Orcs are clearly Melkor’s mockery of the first Children of Iluvatar. Elves. Melkor found them first and corrupted some of them. This is significant because it is specifically said to be the most harmful act of Melkor towards Eru. There are gaps and not everything is fully explained but this is what makes talking about it so interesting.
P.S. Tom Bombadil is an enigma. An intentional one.
P.P.S. The Entwives probably died during Sauron’s scorched earth policies during the War of the Last Alliance but your guess is as good as mine.