[Ulvenok]

Bombadil was expressly said to have no power over the Ring, which was the same as saying he had no power over Sauron.

Note that the ring had no power over him either, so by that sort of reasoning Sauron would have no power over Tom either. However as important as Gandalf's words are on Tom, they are clearly different and Tom making the ring dissapear completely makes it seem even though Gandalf says otherwise, Tom had some sort of influence over the ring.

It was said at the Council that Sauron could not be defied by Bombadil alone.

His reason for not "being able" to take on Sauron alone does not have to be because he has not the ability to do so. He won't leave his old forest either, doesn't mean that he cannot. Also they say Tom wouldn't understand the importance of the ring, one could draw parallels between this phrase and Manwe's unability to understand evil. Any ordinary maia would understand the importance of the ring and also be influenced by it. Tom seems to be a more abstract entity more like the great powers than any other maia we know of.

but could not is the more important point.

Maybe the reason he could not is the same reason why Oromë "could not" ride into mordor and take care of Sauron.

The Ring had no power over Tom because he had no inherent desire for power he did not possess already; he was satisfied with his state of affairs. Gandalf said Bombadil was "his own master", which to me is simply a statement that Tom's purpose in Arda was unique, not necessarily a measure of his inherent power.

Why use the word "master" then.

Noun

A man who has people working for him, esp. servants or slaves.
A ship or boat with a specified number of masts: "a three-master".

Adjective
Having or showing very great skill or proficiency.
Verb
Acquire complete knowledge or skill in (an accomplishment, technique, or art).

Since none is working for Tom, Gandalf's use of master can't be a noun in which case it's an adjective which seems to suggest that it indeed was a statement made on his inherent power, also Goldberry saying that "he is" also has to be taken into account maybe more so than what Gandalf said. Purpose really never plays into it in my opinion, he seems to have little purpose if any at all.

That's all I was saying. Tom and Ungoliant seem to be similar in that both are obviously beings with great spiritual power, each having motives and purposes that appear to have been their own. Ainu who entered Arda separately from the Valar and Maia would seem to fill the bill.

The ainur were both maia and vala. What I'm arguing for is that both have shown feats of "spiritual power" that exceeds any other maia we've heard of in the books therefore have to be more vala than maia. However the valar are specifically those who entered arda, there are none else which means labeling Tom and Ungoliant is tricky.

I still recall reading at some point Eru sending "other" spirits into arda before the valar or at the same time as them that are not accounted for. I don't think it's the quote that you gave me. I read it somewhere at some point but I can't remember.