Re: Valley of the End
Most of everything you said just makes sense.
From a narrative aspect, taking everything back to the Valley of the End would close the loop created so long ago when Sasuke first left. I'm pretty sure that river was where Madara and Hashirama first met, and they returned their to have their big showdown. Generations later, Naruto and Sasuke, the modern embodiment of Hashirama and Madara, first demonstrated their abilities (like Madara and Hashirama's rock throwing). Since then, they have grown to be some of the most famous shinobi in the series, and it only makes sense for them to go back one last time.
There are some issues with this, though. I don't think the Valley of the End would remain intact if they fought there. Like Hashirama and Madara, Naruto essentially has the ability to completely alter the landscape. This means that Sasuke will need to be on a similar level to even challenge Naruto. Therefore, they would probably completely level the statues in the valley if they fought there. Perhaps this could signal the destruction of the old shinobi hidden village system in favor of whatever new model that will emerge after the war, but I don't think completely erasing the monument and the Valley of the End is necessary when Naruto and Sasuke can literally fight anywhere.
As for the position of Hokage, I'm pretty sure that Sasuke isn't even a candidate at this point. He, along with his entire team, is still considered a criminal and a rogue ninja, and I don't think anybody really trusts him. Furthermore, I don't really think that Sasuke was really that important in bringing down Obito.
Just like Sakura, Sasuke serves as a foil for the main character. Thus, Kishi can't completely ignore his development or leave him out of the story, but his biggest contribution to the war effort was bringing back Orochimaru. Orochimaru pretty much did everything else. Even his participation in the battle with Obito seemed a little forced. For example, the sword used against Obito was supposedly from Sasuke's Susanoo, but it was apparently composed of Naruto's strong will and determination, and thus Sasuke merely provided the form of the weapon, while Naruto supplied their overall form, the attack used to break through Obito's defenses, the necessary (will) power to land the final blow, and the chakra and method to pull the Tailed Beasts out of Obito.
Finally, pretty much everybody has been saved by Naruto at some point or another during this war. He saved the members of the Alliance countless times, he saved each of the more important supporting characters from some sort of peril or hardship at some point in the series, he even is the catalyst that eventually reunited Sasuke with Itachi. Finally, he will likely be the motivation behind a complete restructuring of the hidden village system.
In other words, I think that something else will fuel Sasuke and Naruto's final duel. I have no doubts that it will still happen, but I really don't think that they really a reason to fight other than out of competition. Sasuke currently doesn't want to destroy the Hidden Leaf, and after the battle with Obito and the coming battle with Madara, even Sasuke has to realize that such ideas are ridiculous. After all, petty little squabbles, jealousy, and vengeance are what led to his clan being killed off in the first place. I think that Sasuke is going to be able to learn something from fighting against Madara (such as Perfect Susanoo) and finally get his much needed power boost. But instead of vying for the position of Hokage, I could just as easily see Sasuke admitting that he is not suited for being a Kage, starting his own village (a new Sound Village perhaps), or taking on a supporting role to Naruto. However, they will continue to compete with each other, and will agree to meet for one last showdown. They fight until they each only have their basic attacks left. And with a Chidori and Rasengan, they clash, and Kshi ends the series.