Okay, well I think that rab is 2 people. The reason for this is, dumbledore took harry with him. Not because he wanted to show harry, but because he needed him. He could not take the locket by himself, thats why harry kept forcing him to drink the potion, if it was not for harry they would not have gotten the locket. So rab must be 2 or possibly 3 people.
Now i don't get one thing about THE CAVE...There is no answer to this...Only theories.
So DD and HP go in right? They get to the potion right? They can't touch it or make it dissapear.....BUT! There is one way that it could have been done that it didn't mention in the book. Why didn't DD take his Crystal goblet, fill it, and then empty it out into the water. What said that he had to drink it????
I have a theory about something that might happen in the next book. During book 3, DD says to Percy when everyone is sleeping in Great Hall
"No Dementor shall cross the threshold of this castle while i am headmaster."
I think that in the next book we might hear of some Dementor attacks on Hogwarts. Just a theory though