haha, this is what i hate on nb, people jumps to conclusions after 1 misunderstood text, im sure he meant that he wanted to be 100%sure that obito,kakashi and rin are all alone, a ninja of minatos caliber would have noticed change in obito and would have put end to an madaras plans. + at that age minato would have kicked madaras old ass quite easily and save the day.