Your explanation of Snape sparing Harry’s life requires some information we don’t have yet. Believeing the opposite (Snape is still on the side of good) only requires looking at clues already planted by JKR:

1.) Dumbledore freezing Harry – truly a stupid move if Dumbledore wishes to survive. He has just completed a dangerous task where Harry was his active partner.

2.) The argument between Snape & Dumbledore overheard by Hagrid.

3.) The similarity in JKR’s chosen language (hate/revulsion) on both Harry’s & Snape’s faces when they are harming Dumbledore.

4.) Her decision to withold Dumbledore’s reason for trusting Snape to Book 7.

5.) Snape’s protection of Harry from other Death Eaters. Snape’s saving Harry’s life on previous occasions. Snape’s giving Harry advice even while they are fighting. The lack of any evidence Snape did anything during the attack on Hogwarts except kill Dumbledore.

6.) Snape’s rage at being called a coward.

All of these require no additional hidden information to fit neatly into the conclusion that Snape is still working against Voldemort.

So I’ll work on the premise that Snape did kill Dumbledore, and did so against Dumbledore’s will.

Dumbledore’s plea to Snape can be explained as a plea for Snape’s sake, not for his own. He’d realized that Snape was on the verge of irreversibly going over to the dark side, and wanted to prevent that. Snape not killing Harry could be explained by Harry still being under some sort of protection, if only that Voldemort wants to personally kill Harry.

My observations (and bear with me Russell, because you’ve read some of these on your own blog):

1.) Harry is unlikely to be a Horcrux. Because of the Prophecy, Voldemort knows he must destroy Harry. Why would he make a Horcrux of someone he knows he must kill? Too illogical.

2.) Lily can’t be Voldemort’s sister. In the scenario you outline, it would make her older than Voldemort and we all know Voldemort attended Hogwarts prior to the Potters.

3.) Snape knew Harry had his old Potions book. He knew it before Harry cursed Draco. This is stated bluntly during the book’s conclusion. Yet, after Harry hits Draco with a very serious curse, based on the Dark Arts, what does Snape do? Gives him a couple of detentions – that’s all. No talk about expulsion. No reporting it to McGonagall or Dumbledore. Also, Snape knows darn well that Harry is still in possession of his Potions book and he lets Harry keep it. Clearly, if he were on Voldemort’s side, he would have forced Harry to surrender the genuine article. My guess is that Snape left his old Potions book behind precisely for it to fall into Harry’s hands. As it’s still safe in the Room of Requirement, I trust we’ll see it again in Book 7.

4.) I think there is great significance in the fact that Dumbledore froze Harry as Draco disarmed Dumbledore. First, one of the major themes of HBP was Dumbledore making Harry an active adult partner – not a child who needs protection. He just took Harry on a perilous journey to retrieve a Horcrux. At that moment, Harry is under an invisibility cloak and in no immediate danger. If Dumbledore was looking to defeat the Death Eaters, why immobilize the greatest weapon he’s got outside of himself – especially at the cost of losing his wand? No, Dumbledore knew that Harry would attempt to save Dumbledore and that would have spoiled Dumbledore’s plan.

5.) Dumbledore’s sacrifice is perfectly in keeping with his character. If he triumphs over Draco, both Draco and Snape both die – Draco at Voldemort’s hands and Snape due to the Unbreakable Vow. I’m sure Dumbledore would chose to die himself (as he is unafraid of death) rather than see those two die. His concern for Draco is underlined in the conversation he has with Draco immediately prior to his death.

6.) Snape tells Narcissa and her sister that he hasn’t killed Potter previously because Potter was Dumbledore’s favorite and it would have exposed him as Voldemort’s man. This is absurd because Snape has done considerably more that “not kill” Harry. He’s saved Harry’s life in Book 1 and Book 3. If he really were Voldemort’s man, all he would have had to do is let Harry die

Now for some observations from the “technical,” writing point of view:

7.) Rowling has demonstrated again and again that she is a skillful mystery writer. And she adheres to Rule # 1 of Mysteries: you must give the reader enough clues for them to solve the mystery themselves. Why would she tell us that Snape’s hand “twitched” when he was asked to take the third part of the Unbreakable Vow (the part where he vowed to kill Dumbledore if Draco couldn’t)? Similarly, Harry & Snape’s exchange about cowardice is there for a reason.

8.) She includes the information that Dumbledore & Snape were arguing about something that Snape didn’t want to do – yet, does not reveal the actual content of the argument. Clearly, that is coming later.

9.) Similarly, and even more importantly, she has been teasing us through several books about Dumbledore’s “ironclad” reason for trusting Snape. If this were the “final reveal” about Snape’s true character, I think she would have told us the reason first, making Snape’s treachery even more shocking. Revealing the reason after the betrayal is very anti-climatic.

10.) Let’s also look at Rowling’s language in two passages. The first occurs as Harry is force-feeding Dumbledore Voldemort’s potion:

“‘You’ve got to keep drinking, remember? You told me you had to keep drinking. Here…’ Hating himself, repulsed by what he was doing, Harry forced the goblet back toward Dumbledore’s mouth …” [P. 571 (US)/531 (UK)

The second occurs less than 30 pages later as Snape prepares to unleash his Avada Kedavra:

“Snape gazed for a moment at Dumbledore, and there was revulsion and hatred etched in the harsh lines of his face…” [P.595 (US)/556 (UK)]

Is it a coincidence that within very few pages, JKR chooses the same root words “hate” and “repulse” as both Snape and Harry are forced by circumstances to harm Dumbledore?

11.) We have all agreed that Dumbledore would not be pleading for his life. Dumbledore had been very anxious to have Snape at his side (he mentions to Harry to fetch Snape and only Snape and to tell no one). As Dumbledore had total confidence in Snape, why would he think Snape was going to harm him? Snape doesn’t burst onto the scene exclaiming, “Guess what, Albus? I’ve joined the Death Eaters!” At the moment he appears, even Harry thinks he’s there as part of the Order. So why the plea? The ONLY explanation that makes sense is that Dumbledore wanted Snape to kill him and he already knew (thanks to their previous argument) that Snape really did not want to do it.

Anyway, that’s my story and I’m sticking to it.

A curious element of this whjole book is its emphasis on Snape’s powers of healing: he instantly heals Draco’s wounds after Harry uses the Sectumsempra spell on him, something that strikes Harry forcefully enough that he recalls it later. But then, the Sectumsempra hex was invented by Snape himself — could it be, then, that when Dumbledore seeks Snape’s help after encountering Voldemort’s Horcruxes, that is because some of Snape’s own magic went into them? (Probably not, since otherwise Dumbledore wouldn’t Dumbledore have gotten more information about the Horcruxes from Snape? Still, it’s curious.)

Very interesting speculations. . . .

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Dumbledore wasn’t simply in physical pain in the cave. As I was reading that passage the first time, two things struck me. First, Harry’s willingness to keep giving Dumbledore the potion seemed too easy and not exactly in character. Second, Harry and Dumbledore seemed to be talking past each other. Dumbledore is saying odd things like, “I don’t want,” “I don’t like,” “Don’t make me,” “Make it stop,” “I can’t, I can’t, don’t make me.” So far, it’s weird dialogue, but basically consistent with a guy that doesn’t want to drink a potion, but is being made to.

After that, his words make it harder to sustain that interpretation. He says, “It’s all my fault, all my fault; Please make it stop, I know I did wrong, oh please make it stop and I’ll never, never again….” He says, “Don’t hurt them, don’t hurt them, please, please, it’s my fault, hurt me instead….” “Please, please, please, no…not that, not that, I’ll do anything….” And then his last words, in all caps, “KILL ME!”

After he partially recovers, he repeatedly insists that he needs to see Snape, in such a way that it seems unlikely that Dumbledore (who shows few signs of self-interest in the series) is doing so because he wants or expects to be healed by Snape.

My loose theory is that Dumbledore was seeing something while he was drinking the potion: perhaps what was happening at the school contemporaneously or in the near future. That would explain his sense of guilt, his desire for “them” not to be hurt, and ultimately his command to “KILL ME!” He’s pleading for an opportunity to intercede, to offer himself as a sacrifice to save others (e.g., Malfoy, Snape, the underprotected students at Hogwarts, etc.).

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One of the things that makes this such an intriguing possibility to me, as I say in a comment right after my friend’s, is that leaving behind as a final ward to protect the horcrux a potion that would give the drinker a foretaste/vision/experience of their own death fits Voldemort perfectly. His greatest fear and pre-occupation is his own death; surely he would assume that there could be no better guard than putting people through what their own death would involve. Anyway, make of it what you will.

1) What kind of potion did Dumbledore drink in the cave?

2) Dumbledore said that the defense of the horcrux was good – that it needed two people to beat it. Who did RAB take with him when he stole the real horcrux?

3) If RAB drank the potion (or his companion) why was there some left when Harry and Dumbledore got there?

Reasons that Dumbledore is alive:

1) He said he could fake a death.

2) Harry thought Dumbledore looked like he was sleeping.

3) Dumbledore is the greatest wizard of his time.

Why Lily isn’t Voldemort’s half sister: wrong generation. More likely she is related to the Blacks, (and therefore Snape and Narcissa).