I think people are too quick to jump and scream plothole. Kishimoto is very good at keeping up with his story and tends to plan things out well in advance. He just LED us to think that the Third Hokage had stopped Minato's summon but if you followed the story you could actually catch onto the truth about the Reaper. Kishimoto later likes to spell it out for the dumb people.
Where does it say that you can't prepare the body first, and do the initial summoning of the soul later? From what i've picked up on, just like any other summoning technique, the summon itself would take up chakra. Once the summon is stored, the summoner can call upon it for free any time he wants. Most likely Orochimaru just got cocky and assumed he could summon all three Kages without testing it beforehand. Orochimaru did seemed surprised, "Huh..? It failed?"
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This here explains that first you need the DNA to SUMMON the soul from the Pure World. Nothing can keep you from storing the corpse and the summoning scroll inside one of the sarco****us and finish the actual summoning later. Notice Kabuto summoned Madara that way as well. He showed Madara's remains in a dormant state to Tobi, and only actually summoned his soul into the body much later. Madara was surprised that Impure World Resurrection was used to bring him back rather than the Rinnegan.
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If anything, this shows us that there might be different ways to work on the summoning depending on what the caster wants at the moment. The summoning of Madara happens AFTER and works the same way as the Orochimaru variation, which means Kishimoto did not make a mistake.
