[Raynor]

But whatever she is, she came from outside Arda and she's deeply scary.

I still am not sure as by what criterion you would define her as not earthly; embodied maiar, the Incarnates (dwarves included) and the Ents have the same status (though they might differ in power); their fea comes from outside Ea and their hroa is made from the same matter. So, an earthly power would be...?

before they turned up, Morgoth himself was grovelling beneath her manifold legs.

Yeah, after he put forth a great deal of his power, fed her with many jewels and after she sucked dry the wells of Varda and the Two Trees.

Back to the Ring.

I agree with the idea that there is no point in hiding the ring; as far as destroying it, I guess only the power of Arien is sufficient enough(even Melkor feared her) and she and Tilion are the only powers that still are directly related to Arda. How to get her involved is an entirely different matter .