What's the future of North Korea ?

[epronovost]

1) North Korea doesn't have many nukes but enough to destroy Seoul if they used all of their nukes ? Or would they aim for Tokyo ?

They have well enouh nukes to destroy Seoul and a few other cities too (they have about 15 of them). What they lack is a viable way to deliver those nukes (missiles) though this is developping. Of course, you simply cannot use nukes unless you plan on self destruction.

How can the Koreas be reunified ?

At the moment they can't. The collapse of the North Korean regime would trigger the largest humanitarian crisis in the world and a massive migration of million of starving, uneducated and affraid North Korean toward China and South Korea. It would be a crisis like we have never seen before and an order of magnitude more complex than the reunification of Germany.