Greetings!
I finished the book for the second time last night (made for a very late dinner.)
I believe I found proof that Dumbledore was pleading for Snape to kill him, not to save his life:
"...at that precise moment, the door to the ramparts burst open once more and there stood Snape, his wand clutched in his hand as his black eyes swept the scene, from Dumbledore slumped against the wall, to the four Death Eaters, including the enraged werewolf, and Malfoy.
"We've got a problem, Snape," said the lumpy Amycus, whose eyes and wand were fixed alike upon Dumbledore, "the boy doesn't seem able-"
But somebody else had spoken Snape's name, quite softly.
"Severus..."
Okay, now, fellow sleuths. Why would Dumbledore have been pleading for his life when he spoke Snape's name? At no point in the passage does Snape appear to be threatening Dumbledore in any way. If Dumbledore believed Snape to be on his side, it wouldn't occur to him that Snape was about to kill him, unless it had been arranged beforehand.
Ha! Back to OOTP, to figure out what clues I missed leading into HBP.
You are so fast. I am reading it really, really slow this time. They are just making their way to Slughorn's.
You all have me just about convinced that DD's death was prearranged with Snape.