So the kages are more important than the juubi and madara whom are about to solo the NV?
I think hasi's priority should be the battlefield, tobirama was on rage to fight madara, sasuke and his crew don't care. It doesn't make sense to me, specially after tsunade's words and considering the circumstances!!
Of course with the 2nd being an amazing sensor, Karin, being a sensor, and the 4th, there is no way they wouldn't notice 5 kages on the brink of death. Minato was sent ahead to help the shinobi force while Taka and the 1st Healed the Kages, Karin's ability to heal, and the 1sts.
This is the reasoning behind why Minato was first there, he was sent ahead.