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I take from this that you are assuming that the security guard was the only person in the Atrium? In which case the death eaters could have gone ahead, Imperiused the guard, sent a messenger back to Voldemort saying that the coast is clear, leaving the way clear for Voldemort to get into the Ministry.
Yes, I am assuming that the security guard was the only person in the atrium, but if people were working late in their offices than they could have walked through the atrium at anytime and if they happened to see voldemort walking through than that would be very bad for the death eaters. If they were walking through the atrium and saw harry walking through than it wouldnt be a big deal, if anything harry would get in trouble.
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I don't really understand this - Voldemort could have still have planted the vision in Harry about Sirius, luring Harry there but guaranteeing him getting the prophecy (as he could take it himself), and also pretty much guaranteeing him a captured Harry, (seeing as his magic is, of course, much superior to Harry's) so there is a much better chance of getting Harry if he went there himself.
Well that is exactly what he did. He planted the vision in harry, and he showed up at the ministry to kill him. He sent harry to do his dirty work and then he was waiting for him at the exit to kill him until dumbledore showed up.
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I still think he would have gone there himself - as we can see from the previous and future books, Voldemort likes to add a personal touch when dealing with Harry Potter eg in GoF he insists on using Harry for his regenration despite being perfectly able to use any other wizard. I think if Harry Potter was going to be there he would have wanted to be the one who captured him. After all, he has defied him four times before, I think that he wouldn't have wanted to put the responsibility on his Death Eaters after seeing how good he is at getting out of tight spots. I don't think he would have wanted to run the risk.
As i said above, he did end up going there himself. You are right about the personal touch which is why i am sure he told his death eaters to capture but not kill harry. And about voldemort seeing how good harry is at getting out of tight spots. Im sorry but i think you are off base here, voldemort refuses to believe harry is anything except very lucky all the way up until he dies in DH. Voldemort underestimates harry because he is just a teenager and he refuses to believe that harry is anything special