I have two questions :
The first question is whether Orochimaru's third coffin failed to summon or Hiruzen stopped it
The second is what happens when you use RDS through a shadow clone or any sort of clone ?
1st: Hiruzen used the Technique Neutraliuzer to stop the coffin midway. If the coffin still went up it still would have been denied because Minato's soul was still in the Reaer
2nd: You seal parts of your soul away.