[Aiwendil]

Do you mean, he thought there wouldn't be some great "final battle" at the end of Middle Earth?

Whether he abandoned the Dagor Dagorath is debatable. It is definite that he abandoned the Turin prophecy - we know this because he replaced it with a prophecy wherein Turin returns at the end of the first age to slay Ancalagon (instead of Earendil). This revision was unworkable for the published Silmarillion, so Christopher left it out.

In HoMe IV, it also says that Túrin and Nienor became of the Valar...so he didn't really need to "return" from anywhere.

True; but the prophecy occurs in QS from HoMe V, wherein (unless my memory fails me), Turin and Nienor do not become Valar.

Sauron was completely destroyed, because he had poured his power, or life-force, into the Ring.

I'm not sure whether this is precisely true. I can't recall any specific reference at the moment, though, so I can't really argue. At any rate, I think you are right that Sauron was defeated in a far more profound way than was Morgoth. I always suspected that there was some connection between the mystery-shrouded defeat of Morgoth at the end of the world and the central analogy of "Morgoth's Ring". If Melkor had indeed dispersed his power through Arda just as Sauron had focused his in the Ring, then it follows that Melkor cannot be finally defeated unless the world is destroyed, just as Sauron cannot be so defeated until the Ring is destroyed.

Gurthang could just be reforged in the conventional sense (i.e. taken by a smith and reforged) or in the spiritual sense (another sword made of a meteor? that Túrin named Gurthang in memory of his original sword).

Another somewhat outrageous possibility: perhaps whatever intelligence was in Gurthang actually had some kind of spirit or fea, and later possessed a new blade. Of course, I don't believe that for a second.

but later all this was changed into the silmarillion. so i am guessing this was all in a super early form.

Exactly. The concept you described was pretty definitively abandoned, as we have very firm evidence from later on that Men make a stop at Mandos, then depart from the world.