Aren't you forgeting something? Kimmimaru or w/e his name is killed the FOURTH/4th Kazekage. Oro could have summoned him...however there is no question in my mind as to the third coffin that orochimaru tried to summon that the 3rd hokage stopped from being fully summoned. it was minato, the 4th hokage, yondaime.
Besides, im sure he knew that Minato is sealed in DDS. Even if he already left Konoha by the attack of Tobi he still had plenty of spies to know about Minatos death.
Only way that could be 4th Hokage in the coffin is if Kishimoto originally didn't planed for Minato to be sealed in DDS. It could be smth he thought of while writting the manga.
There are some things in manga that doesn't make sense or that aren't thought through. Like for example Tsunade's parents. Her father/mother is the son/daugher of the Hashirama Senju and Mito Uzumaki, and the father/mother of Tsunade herself. He/she has two of those incredibly powerful bloodlines and yet he/she is a nonfactor not even worth mentioning in the story. That's bullshit, that person should have been in one league with Madara and Hashirama.