As said in the first post there may be spoilers here so if you don't want to read one look away now.
We don't know exactly how kakashi obtained the mangeko Sharingan yet but it is a fact that it was not the same way Itachi got it, also Kakashi uses a seal to activate his mangeko sharingan. I also think that the forth is Narutos dad but I also think that he is the leader of akatsuki, I know the moe he used against the Kyubi was ment to kill him but I don't think it did I have my reasons why
Also who would you rather fight, a giant demon fox that has practically the largest chakara in the world (thats what I think anyway) or a teen age child who can barely control the power of the nine tails. If he was the leader of akatsuki this would make perfect sense as it would make their mission easier