I think Dumbledore had discussed at length with Snape what Draco was up to. Snape didn't really know what Draco's actual plan was. That was even said in the book. Draco refused to tell him because he thought Snape wanted to steal his glory. He only knew Draco was supposed to come up with a way to kill Dumbledore. I think Dumbledore knew that Snape had made the unbreakable oath in order to continue his ruse as a Death Eater, and I think when he said, "Severus," in a pleading tone, he was trying to convince Snape to do what he had to do and continue on because his job as a spy was just too valuable to the Order. Maybe he was worried Snape would back down when it actually came to ending his life. I still believe that Dumbledore was right about Snape and that Snape is still on the right side. I will be very surprised if he truly is a bad guy. He could have not only killed Harry very easily, but any of the other kids as well, but he didn't. He only stupified Flitwick. Wouldn't a true Death Eater have killed or at least severely maimed or scarred them? The Death Eaters enjoy making their enemies suffer, yet Severus showed none of that.
I'm more curious about where she goes with Draco in the storyline. WIll he end up joining the fight against Voldemort, or will he just succumb to being a Death Eater?