Why take a body? Why make a Ring?

[Raynor]

But Angband would be a living Hell, a very different thing to a hell for the soul or spirit. I also tend to view Mandos as more like purgatory

Mandos as a purgatory is something I can live with ; there is no suffering coming from outside, on the contrary, given the Nienna factor. Maybe even purgatory is strong a word.

Is it dissipated?

I doubt that:

Originally Posted by Letter #211

The indestructibility of spirits with free wills, even by the Creator of them, is also an inevitable feature, if one either believes in their existence, or feigns it in a story.

In fact,can we call it a Fea if we are talking about one of the Ainur? Or do the terms Fea and Hroa only apply to Incarnates, i.e. Elves, Men, Dwarves? In the Osanwe-kenta Tolkien makes a clear distinction between the Ainur and the 'Incarnates'.

I believe the difference lies only in orders of magnitude; in the text you refer to it is also stated that:

Pengolodh says that all minds (sama, pl. samar) are equal in status, though they differ in capacity and strength.

and from the context it is understood that he referrs to ainu-level and Incarnates too. The term fea is translated as spirit; the valar and the maiar are also referred to as spirits in the Silmarillion.