[denethorthefirst]

I am also of the opinion that the One Ring allowed Sauron to manipulate the Morgoth Element. While this theory is not directly supported by Tolkien (but then he never directly explains how magic works in his world) it makes sense for the following three reasons:
1. Instead of a merely technical ring-controlling tool the One Ring becomes much more mysterious, powerful and dangerous.
2. It makes Sauron a more successful and cunning villain: his ring scheme did only partially fail because the primary function of the ring was to control the Morgoth Element.
3. And last but not least: the theory explaines how Sauron (in a world that is entropic: everyone becomes lesser and loses power over time, even the Valar!) managed to become more powerful with the ring even though his ring scheme failed and he effectively only dominated the nine: he appropriated the dispersed power of his former master.

Tolkien writes that Ainu, once they become fully incarnated, can be killed like the children of iluvatar and are then not able to rematerialize, they stay "dead" (of course they cannot really be destroyed, their spirits are immortal). Tolkien speculates that maybe the disembodied Melkor could achieve reembodiement in the very distant future (but as Tolkien states he is a special case because of his former greatness); There is, besides Sauron, only one case of an incarnated Ainu that was killed and "came back": Gandalf, and that required an intervention by Eru! Those reappearing Umaiar you mentioned are from relatively early in the timeline and were probably only "clothed" and not incarnated. A merely clothed Umaia that is robbed of his form can easily reshape. I guess that during the War of Wrath most of the Umaia had become incarnated and a Balrog that had been killed during that conflict was effectively "dead".

Concerning Gondor: I actually find it quite unbelievable that a Steward (or another house for that matter) would claim the kingship, it goes against the whole upbringing and cultural/national identity of Gondor and the stewards: Gondor is the land of Anarion, a Gondor that no longer is the land of Anarion is no longer Gondor. In order for that to happen the whole identity and culture of the place would have to change (can anyone imagine a monarchic USA, an anti-Semitic Israel, a Protestant Italy or a republican Japan? Same thing.) You're right in that there probably weren't "A lot" of great houses that were more noble than the House of Hurin and had connections to the House of Anarion, but there were at least a few of them and the house of the stewards was not on top, neither socially nor economically.
Maybe if a Steward achieved some otherworldly success that rivaled Elendil he would have a chance to start a new royal line: defeat Mordor, slay Sauron in single combat, significantly enlarge the realm, renewed contact with the elves plus a marriage, etc. - but that's not very likely is it?