Maybe Madara forced the kyuubi out, and it took over Kushina, similair to what happened to Gaara. And perhaps the 4th did have the power to kill the 9- tails but instead sealed it so he didnt have to kill Kushina (and instead seal her in her son). Or its possible giving birth allowed the fox to take over and not Madara.

I like the idea of Naruto being the direct descendant of the sage of six paths. Fuck Sasuke and his eye hacks.