Having one character, whom you are trying to inveigle into the enemy clan, "kill" one of your own side in a quite dramatic and public way is, of course, a classic literary device. Having just read the above website and since then re-read HBP with the specific mental note that Dumbledore WASN'T dead, but it was all a set-up, things perhaps make a lot more sense? [Like watching "Sixth Sense" already knowing the twist.]
The site mentions things like the Avada Kedavra performed by Snape had the wrong effect to be real and would have been easy to fake. Perhaps more importantly, if he hadn't performed some fake spell, assuming this theory holds, one of the half-dozen "real" Death Eaters on top of the tower WOULD have performed the real spell. Which would have been a disaster. And one of the last meetings with Harry, Snape really bellows at him "I am no coward". Which under this theory would be true - he has risked the most and technically been the most courageous, working right under HWMNBN's nose as a double-agent. Okay, there is still personal hatred for Harry and his family, but Snape would appear to still be on-message. For example, we've never had Dumbledore say what the big secret is for why he trust's the man. I'm predicting Harry is going to have to apologise, big time, to Snape in book 7, or even have to save his life.
Dumbledore could always see Harry, even under his cloak and could clearly sense Draco's approach. He could have petrified the boy as easily as he did to Harry but knew he didn't need to. What he needed instead was the most reliable witness to his own "demise": his star pupil, who incidentally would perhaps not spot the fix-up due to emotion (though it also fooled the Death Eaters) and more importantly might need to have that emotion as a back-stop during some confrontation with HWMNBN in book 7.
If the whole exercise is considered as: "how would I go about writing it IF Dumbledore's death was to be faked", then what is actually written is about as close as I suggest you'd get.
