First off the one thing that has always remained a constant, is that Dumbledore is NO idiot, the greatest wizard, yada yada yada.....
So WHY would he be Fooled by Snape? Not likely.... I DO believe they had some sort of Unbreakable Vow, its definitely NOT what Dumbledore explained to Harry, there is MUCH more to the story of why he trusts Snape and I figure we'll find out in Book 7.
With that in mind, we know he already knew the Malfoy plan. If Snape was Bad, he wouldn't have let Dumbledore onto that one, b/c it would give him chance to escape or foil the plan.
Also, nobody trusts Snape, and Dumb. is the only one that does, so it would be really odd that he backstabs the only person that trusts him.
Now it seems the trip that Dumble did with Harry may have been a wild goose chase b/c soemone had already found the Horcrux. I believe Dumbl knew this already, OR at least that once he found it, he knew immediatley it was NOT the Horcrux. WHen Harry saw it, he already noticed the S was missing, etc....
Dumbledore would've grabbed it and immediately known it wasn't the real one.
I believe he drank the potion b/c it was the "draught of the living death" mentioned in Book 1, page 137-138 which during that potion lesson, Snape also mentioned the "bezoar" - How convenient that the bezoar was used in this book......so I belive the "draught of the living death" was also used. I believe that was Dumbledore's MAIN reason for going over there in the first place, not the Horcrux. That's why Dumbledore made Harry promise to make him drink the entire potion, b/c he needed to make sure it had it's full effect.
Now he was "supposedly" so weak, but yet had enough strength out of nowhere to create the fire to shield Harry from the Inferi.
So if you believe this theory, this next theory will make sense.
Dumbledore upon getting back to the castle, and really weak at this point, told Harry to call for Snape. Insisted on it. Then of course, the whole Malfoy thing happens before Harry gets a chance. Dumbledore immobilizes Harry, yet getting weaker by the minute, the Death Eaters all come, and Snape rushes in and kills Dumbledore. And realize Snape was really RUSHING to get to the scene. Now if you think about it, Dumbedore the greatest wizard, yada yada.... doesn't need his wand, and even in the weakest state, could do a nonverbal spell like the best of them. But he didn't or couldn't or whatever.
So I believe, that Dumbledore took that potion, to make him SEEM dead. It takes a certain amount of time to take effect, and that's why we saw him getting weaker by the minute. He also knew that he made the deal with Snape to do the Aveda Kavada curse on him, and Snape didn't want to b/c they were having an argument over him doing it. He immobilized Harry so Harry wouldn't mess up the plan by trying to save him, b/c of the potion it would look like he had died anyway, and that would ruin the plan of making it look like Snape killed him (he already knew Malfoy would NEVER have the courage to kill) But that's why Snape had to HURRY to do the curse to have the Death Eaters believe that he was indeed the one who finished Dumbledore off, to make him truly trustworthy of Voldemore, and also not break his Unbreakable Vow to Malfoy's mom.
But what I think we've all forgotten, is that when they first mentioned the Aveda Kavada curse, and I can't remember which book, but they said, "It only works if you MEAN it"
Snape didn't want to kill Dumbledore, so the curse wouldn't really kill him b/c Snape didn't MEAN it. But with the potion, it looks like the curse did kill him afterall. Which I think the end when they see Dumbledore's portrait he is "slumbering" I think that was a major clue. And of course, you all are right on when you say Dumbledore wasn't begging for his life, but for Snape to carry out the plan and do the curse.
Which why in Book 7 Harry will have more of a vengeance to finish Voldemore. that's why Dumbledore gave him all that information in the memories, b/c it's a job Harry will have to do on his own. But I don't believe Dumbledore is really dead at all. JK has been saying all the stuff about she doesn't belive in bringing back people from the dead, and kids have to know about death, etc... but HOPEFULLY she has a heart and that is all a coverup to make us believe when the people in the book die, they really do, I believe Sirius and Dumbledore are definitely alive.
Speaking of Sirius, on page 11-12 Fudge says "But Black's by-the-by now"
JK has never used that phrase to say someone's dead. Don't know what it means, but I think it's important.
About the locket Horcrux. In book 5, there was a locket in Black's mansion that the kids couldn't open. That is the real Horcrux, most likely taken by Black's brother thus RAB. I believe Mundungus stole the locket Horcrux, along with the goblet, and who knows what else. JK wouldn't have mentioned he stole the goblet unless it were for a reason, so that will be important in Book 7.
Last (for now) but not least, Dumbledore lets Harry know that Voldemore had wanted the Defense of Dark Arts teacher position, and the position is cursed by him, that's why nobody lasts for more than a year. This is the REAL reason he never had Snape fill the position, NOT b/c he didn't trust him like everyone else thought, but b/c he NEEDED him, and couldn't have anything happening to him. But this year, it was OK, b/c of the plan, he knew it would be their own doing why Snape didn't last in the position, and he would be OK.
Question: Does anyone have a theory on why Dumbledore told the Dursley's they had mistreated Dudley? He said and I quote "the best that can be said is that he has at least escaped the appalling damage you have inflicted upon the unfortunate boy sitting between you" And of course Uncle Vernon was appalled. After the conversation it mentions "Aunt Petunia was oddly flushed"
So there is definitely something strange going on there. Aunt Petunia knows the deal, and was also reprimanded (and I can't remember by who, when or for what) by another witch but it was in one of the other books, and that was another hint that she knew more and was way more involved.
My theory is that (and I admit I'm reaching) Aunt Petunia could've been a witch also, but dismissed it and refused. She married Uncle Vernon, who did NOT know she was really a witch. They had Dudley, and he could have wizard powers, but she stripped him of them, and refused to allow him to be a wizard, b/c as we know Uncle Vernon knows nothing of this.
That could be what Dumbledore meant by Dudley being mistreated, b/c we all know they spoil him silly.
Like I said, I know I'm reaching with this, but it's something to think about. I'd love to hear other theories on Aunt Petunia....
OK, I"ve taken up enough of your space, heh heh.... But those are the theories floating around in my head (I know I must have way too much time on my hands, lol