Re: Possible Plot hole?
A true fan does not look for reasons why the story may not work. A true fan looks for reasons why the parts that are iffy could work. This way, the true fan tries to build up the story and make it work the way it is.
With that said, the only people who said the final coffin was Minato, if I'm correct, were Hiruzen and Kabuto. Now, if Orochimaru had prepared his summons before-hand, then he may have done it while Kabuto was around, though I wouldn't think so; if Kabuto wasn't around when Oro prepared his summons, than neither of the people who claimed it was Minato knew for sure. Also, in the panels you provided, it is said that "The third one ... it did not work." Weather it was Hiruzen or Orochimaru who said this, I'm not sure, though it was probably Hiruzen (though Oro may have had the same thought). As you can see the two coffins that did "work" are covered in shuriken, and yet they still open. Honestly, I don't care what happened in the anime, but in the manga there appears to be (by the panels you provided) no sign of the third coffin; for all we know, Oro could have made a handseal he had done for the first two and Hiruzen was expected a third coffin. And Hiruzen's idea that he stopped the third one is just his interpretation; he was fairly certain that Orochimaru would not make a mistake, as he was an intelligent, though mad, man, and therefore the logical answer was that Hiruzen himself did something. Though now we know that Orochimaru did, in fact, make a mistake when preparing his jutsu.
Now, as I said before, a true fan looks for ways to make what seems off, right again. So, what is very possible is that Orochimaru sealed the three coffins away and prepared the bodies of his corpses within each of the coffins, along with the DNA of the three Hokage he wished to resurrect. Then, as Orochimaru summoned the coffins, he performed Edo Tensei at the same time, thus turning his victims into the Hokage as they were summoned, and thus the fourth's coffin not working because of the last-minute summon. Now, this makes sense for the main reason that Orochimaru is seen placing the controlling tags within each of the Hokage's heads after they were summoned to the field; had he Edo'd them before-hand, he would have either done it at the time of summoning, or the Hokage would have broken out of the coffins and done as they pleased, most likely killing Orochimaru as he attempted to control him and do evil with the 2nd's technique.
Now, assuming that it is correct that Orochimaru knew that he couldn't summon Minato, and everyone simply assumed it was him, then we can go back to the fact that it is most logical that Edo Tensei was performed on the rooftop, and not beforehand, and therefore no one except for Orochimaru knows exactly who Oro was going to summon, nor that it matters anymore because it was stopped by Hiruzen (remember, only if we are to assume that Oro knew what he was doing, didn't summon the fourth, and Hiruzen did, in fact, stop it out of his own power).