1. RAB. I think it has to be Regulus. J.K. has said in interviews that there won't be any major new characters introduced in book 7 *and* that we can figure out who RAB is from what we have read so far (which kinda rules out a new character)
My opinion - it is Regulus. Since Harry owns the house now and all the possesions it will be easy to get that Horcrux...except for the their Harry met in HBP! He will have actually stolen and sold the locket which will make it tougher for Harry to find.
2. Dumbledore - dead or just playing an elaborate practical joke? I vote for dead even though I don't like it. I think he needs to be dead to really leave Harry alone. Harry now has no parents, no god-father, and no surogate father. He must face Voldermort alone.
Of course the evidence of him not being dead is compelling.
Stolen from this good thread
This is the big clue. Think really carefully about the trap guarding the locket. That part of the book sails by quickly, and then we immediately get into the whole Dumbledore dying thing and never come back to it. There are some important things in that chapter, though, that never get adequately explained. Dumbledore tells Harry that "Voldemort wouldn't want to kill anyone who gets to this island;" he'd want to keep them alive long enough to find out how they did it. That makes sense, and the nature of the potion in the basin--which incapacitates the drinker--supports the theory.
But here's the thing that nagged me: If that's the case, then what was the purpose of the "no water" enchantment? Why would Voldemort set up his trap so that a horde of undead killing machines spring to life after the intruder has already been incapacitated?
Voldemort must have realized, like Dumbledore, that two people working together could get around his trap. One person drinks the potion and becomes incapacitated, but then the other could just grab the locket and drag the first person out of there. So he added a twist: whoever drank the potion would be overcome with an overriding need for water, and the only way to get water is to disturb the lake. That wakes up the Inferi, and ensures that any helper not incapacitated by the potion never makes it off the island. But that's also problematic, because the Inferi are mindless killing machines. Dumbledore points out that they would attack even Voldemort if he triggered his own trap (which is why the boat was there). And there's not much point in incapacitating the potion drinker only to let him get torn to bits by Inferi.
The solution, of course, is simple. The potion that Voldemort used to bait the trap doesn't merely incapacitate, it also protects the drinker from harm. At least until they are released from the effects of the potion.
Knowing this, let's hazard another guess: Dumbledore knew about this Horcrux--and the trap guarding it--long before he and Harry went to fetch it.
Very interesting theory!
3. Snape good or bad? I think the simple fact is that J.K. has crafted the story such that we cannot know for sure. We can argue/debate/and guess but she has done such a great, great job of setting up this question that we can't know.
Personally I think he is good. I think he rushed up to the tower to make sure that Draco didn't kill Dumbldore. I think he killed DD to cement his place in the Death Eaters so he could get closer to Voldermort.
We see him continuing to teach Harry right up until the end - From page 603: "Blocked again and again and agin until you learn to keep your mouth shut and your mind closed, Potter!"
The argument that they didn't kill him becuase Voldermort wanted to doesn't hold much water because they could have just knocked Harry out and taken him to the Dark Lord.
So he must have had another reason for not killing and not kidnapping Harry.
Hence I think he is going to be the ultimate double/tripple/quadruple agent!
Now all we have to do is wait a few years to find out!
If during that time you are bored check out http://the-leaky-cauldron.org/
A truly freakish place where they have multiple threads on each of these topics - most of them over 20 pages long!