Well the thing is that Sasuke was never willing to listen to Naruto and he was always running away from him. However, him running away from Naruto can also mean that he was afraid of getting persuaded by him and because of that, by running away, he was hoping to stay unaffected, that alone proves he doesn't have such a hard resolve. However, in their latest battle he couldn't help it and so he passed a great deal of time with Naruto whether he liked it or not. Even if he was planning his revolution all along, at some moments it was visible that he was happy for fighting along with Naruto. Just look at him:
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No matter what he was saying his actions were speaking otherwise.
As for Naruto not offering any plan in return I would agree to some extent but at the same time what he suggests is so ideal that you can't simply put in few words. Sasuke's plan was to become a second Lelouch, preety straightforward but Naruto's answer was too vague. What does it mean mutual understanding and solidarity? For Naruto who wanted to achieve a solution like that is simply too complicated to be explained because it can be achieved in many ways but all of them cannot be explained in a manga. For Naruto to start talk about politics and such would be completely out of character.
I must say that I will be also disappointed if he doesn't even try to specify it a bit.
So to answer your questions:
1) No it is believable because I said Sasuke had the potential to be persuaded easily but he simply needed a little push. The fact is that Sasuke has changed his aims many times and that alone proves that Sasuke does not stick to one and only idea. He's willing to change as long as he thinks that what he does is right, right according to him.
2) Sasuke is an antagonist. As I said above he acts based on his sense of righteousness without caring about him being selfish or selfless. His time during the war proved that he didn't simply become a sweet angel but he simply cooperated with Naruto as means to achieve his own goal which at that point was to become the ruler of the world, Lelocuh style. (With what power he would achieve that prior to his meeting with Hagoromo god knows XD ). So no, he was never the villain, just many people tend to look at things as black and white without taking a deeper insight at one's personality and motives, the latter are what make one a "villain". You can do bad things even though you have a good intent, same happened here.
3) HELL NO! If you examine carefully Sasuke, you will realize a trademark of his personality: his opinion must always correct. Sasuke passed all his life trying to correct what he thought as mistakes. He first killed Itachi to revenge his clan and thus bringing balance where he thought he needed, he killed Danzo for his mistake to kill Itachi, he fought against Obito, Madara and Kaguya to also correct what he deemed as wrong. In a sense you could call that "Sasuke's world order". Just like in these cases, he decided to rule the world to correct hokages' and everyone's else mistakes based on the knowledge he had aqcuired from his personal life. If Sasuke had taken part in this war with the intent of going back to Konoha, it would completely contradict his actions, since Sasuke more that anything wants to pass judgement to those who make mistakes. He did not consider as the cause of that mistake neither Obito nor Madara nor Kaguya but the entire world and so if he wanted to correct the situation he would have to take control of the world.
Hope that helped