First of all, I do NOT think Kakashi getting Susano'O is an asspull. A lot of people of had been theorizing for long time that, if or when Obito's eyes ever reunited in a single body, they would unleash a new power, which, again, many thought would be Susano'O. This idea of Obito's eyes unleashing a new power when reunited was even foreshadowed in Chapters 603 and 666.
Kakashi as 6th Hokage
On the topic of Kakashi becoming the 6th Hokage, this is indeed a very interesting plot point. This would explain that why Naruto is not wearing his Hokage clothing in The Last Movie trailer. It's because he isn't hokage nor will he become Hokage anytime soon. Kakashi will probably rein as Hokage for a good number of years. And I geniuinly believe this is a good thing, as I feel Naruto is not ready to become Hokage, so this turn of events will provide more time for Naruto's growth before he becomes Hokage.
Kakashi maintaining Susano'O
I also think it's understandable that Kakashi is able to maintain Susano'O despite his low chakra reserves. Because it isn't HIS reserves that are maintaining the Susano'O to begin with.... It's Obito's chakra that is creating the Susano'O. In fact, this is why Obito tells Kakashi that the "present" he's giving him [double MS] is only temporary, as, once the chakra he gave to Kakashi runs out, Kakashi will lose both his Mangekyo Sharingan and, in turn, his access to Susano'O.
Temporary power ups
Speaking of temporary power ups, Obito's mentioning of his present to Kakashi being temporary leads me to believe that Naruto and Sasuke's powers from Hagoromo too are merely temporary. Think about it; the method in which Obito gave Kakashi power and Hagoromo gave Naruto and Sasuke power is very similiar. This theory can be further supported by the fact that, in The Last Movie trailer, we see that Naruto has wrapped one of his hands in bandages, the same hand that has his Sun Seal. This is most likely to prevent viewers from knowing whether Naruto loses his powers or not, but seeing as how his other hand has no bandages on it, it would seem like he does. Further more, Hagoromo makes mention of how he never wished for the Rennigan to be reborn into the world again, yet here we have him handing it to Sasuke free of charge. This is a very contradictory statement... unless of course, the Rennigan Sasuke was given is temporary.
This might mean that, at some point in time in the near future, Naruto, Sasuke, and Kakashi will lose their powers and Naruto will revert back to Bijuu Naruto, Sasuke back to EMS Sasuke, and Kakashi to Sharingone Kakashi. I believe they will lose these power ups after dealing with Kaguya, and from there, they will return to the Hidden Leaf, where Kakashi will receive the title of 6th Hokage
In fact, this whole ordeal with Kakashi being Hokage before Naruto, Naruto and Sasuke losing their OP powers, and The Last Movie/New Era Project talk may very well give way to part III, the hints are just pointing in that direction more and more.
Obito Returning from the Dead
In regards to Obito trancending between the Living and dead, again, I belive this was done well. We already know that Obito can travel through world via Kamui, so why not travel between the world of the dead to the world of the living? Also, Obito tells Rin that "Chakra is the power that connects these two worlds together". This indicates that, because Obito's chakra was present in both the World of the living and the World of the dead, he was able to use this chakra in combination with his Kamui to bring himself to the world of the living for a brief moment. This feat that Obito achieved was a one time thing, as he himself stated he wouldn't be able to return to the world of the living anymore when speaking to Kid Kakashi.
Sasuke is Useless
As far as the whole "Sasuke is Useless" thing goes, I'm glad he's getting his ass handed to him left and right. He's such an arrogant person, even told Kakashi and sakura that they're of no use in the current situation, yet they've been doing more than he has. I think it's about time someone stomped his ego flat into the ground, it'll be good charcater development for him; to make him more humble.