Kushina and Minato had Karin kept her birth a secret and decided to keep her hidden outside Konoha. After the Nine tails attack Kushina left Konoha because it was to painful to stay there after her lover died and she could have been told by someone that Naruto died. Kushina reaches where Karin...
Naruto only has Wind I doubt he will get another power up anytime soon but as for second element i would think water so he can be the perfect match aganist Sauske and since his mother came from the land of whirlpools and his last name means whirlpool and whirlpools can be created by mixing water...
He already has enough stuff from his dad like Rasengan, Ninetails sealed same looks same Master that trained him. Besides Uzumaki means Whirlpool and whirlpools are a mix of wind and water so his other element most likly will be Water.
Mifune and Tobi could be the same person just look at the wrinkles under their eye. It looks the same if you compare them the wrinkles under Mifune eye on the last page in the new chapter and the wrinkles in the chapter where Tobi/Madara takes off his mask in front of Sauske. He is at the Kage...