[Kuruharan]

The Secret Fire is not Eru Himself, it is the Middle-earth equivalent of the Holy Spirit. It procedes from Eru Illuvatar & burns at the heart of the created World. I see no problem with the idea that it is, in that sense, equivalent to the 'fea' of Arda.

Well, there really isn't a whole lot to say about this that would not get us waaaaay off the original subject.

However, I will say that I prefer my view because, among other reasons, it saves me from having to explain away the knotty (and unanswerable) question of how Sauron was able to harness and abuse something so intimately tied to Eru when Melkor had not been able to find it at all.

But how could the evil which it embodies - its 'fea' - be destroyed? On entering the Fire it is 'unmade' ie it is totally destroyed. The Ring has this 'dual' aspect - it exists as a physical object & as a locus of spiritual evil. The physical fires destroy the gold, the spiritual fires unmake the evil.

When the evil of the Ring lost its physical house it was dissolved because it had no place to go. It could not return to Sauron because, while it was closely tied to him, it was not quite the same thing anymore. The evil in it had been bound to that form and it stood or fell (metaphorically speaking) with that form. Once the form was gone, so was all of its former power.

I am unclear as to why it is insufficient for the presence of Sauron's own power in Mount Doom to render this destruction possible.

the Stones have a kind adictive quality to them...keepers of the Stones keep being drawn to look into them

This does seem to contradict Tolkien's statement that...

Their use involved no peril, and no king or other person authorized to survey them would have hesitated to reveal the source of his knowledge of the deeds or opinions of distant rulers, if obtained through the Stones.

He also says that the Ruling Stewards did not use the Stones until Denethor II.

I think it is more the insidious influence of Sauron more than an inherent quality of the Stones themselves.