In B/W Alder is the Unova champion, but he is defeated by N which made him champion/ Than Hilbert/Hilda beat N technically making either of them the Champion whether the post game E4 rematch is cannon or not
In the 2 years leading up to B/W2 Hilbert /Hilda apparently leaves Unova to look for N meaning they can no longer be the champion then Iris somehow becomes the new one. Then of course. The MC in B/W2 will beat Iris making them the champion.
I don't think the post game happened. I think MC got word from Looker than N is in a far away land and just left with his/her dragon --- 3DS FC: 4167-5373-4749 http://i1.ytimg.com/u/xsasIWeopo0w8WVLqiXM6A/channels3_background.jpg?v=e0c992