And I will (as with my other wild and not-so-wild ideas) be SO THRILLED if I’m right.

These theories may be absolutely brilliant – but could the brilliance be too clever by half?

It’s still so much fun. And hard to believe that after all these years, most of this speculation will be over in just five days.

First, Slughorn. Assuming that each DADA teacher holds a secret (a little iffy for Umbridge, even accounting for the Dementor attack) I figure he’s already given up his: he’s the one who helped turn Tom Riddle onto Horcruxes, to his eternal regret (and possible damnation). Also, despite the creepy ‘oho!’, it’s very unlikely that Slughorn is P!Dumbledore.
Logically, if Slughorn as P!Dumbledore dies on/falling off the tower, who is the Slughorn who confers with the staff and attends the funeral afterwards? And more importantly, the person on the tower is pure Dumbledore – the grace under pressure, the humour, as well as the compassion and mercy. And as importantly, the scene on the tower is one of the best JKR has written. Would she waste her best lines on an imitation?

Second, Bellatrix as Sybill T. I don’t think Ms. B. is capable of controlling herself to the extent of pretending to be such a housemouse as Ms. S. for any length of time. But then, Barty Crouch Jr also seemed a bit of a loose cannon, yet successfully fooled everyone as Moody for 10 months, so I guess if JKR wills it, it can be. However, the reasoning that P!Sybill spills the Snape as betrayer beans to Harry at just the right moment doesn’t really work for me. I think there is an important dramatic reason for the revelation: Harry needs to get really torqued up against Snape before the end.

Third, Pettigrew as P!Lupin? Sounds like pure speculation, although no doubt wonderfully reasoned.

Fourth: Snape has a secret: he’s a half-vampire. Well, Severus has enough secrets that being a half-vampire could just be filed away under the under “half’s”, including half-blood, half-DE, half OP, half-saint and half-sinner. My point is that he already straddles many worlds, without having another dichotomy wished upon him.

Fifth. Either Dumbledore was a P!Dumbledore, or Dumbledore died at some earlier point. P!Dumbledore is really clever, and there is some evidence for it, but see point 1 above. Not only can you not waste your best lines on an imitation, you can not mess with people’s emotions like that and afterwards say “Just fooling, folks!” Also, logically, whoever P!Dumbledore was, that person couldn’t have been around afterwards. And all the usual suspects were.

However, I do think there was on P! character at the finale of HBP. I think it was Flitwick. Reason? Why would Good Snape stun the real Flitwick when the latter came to tell him that DE’s were attacking? Or rather, why would JKR have Good Snape stun the real Flitwick? To keep Flitwick and the watchers in the hall occupied and out of the way is the only reason I can come up with. Is that a good enough explanation?

I’ll know soon enough.

I’ll only respond to one point here, as I’ve already responded at length to the P!Dumbledore theories elsewhere, and find P!Lupin to be little more than baseless conjecture.

I’m not convinced that it would have been impossible for Trelawney to have known Snape was the snoop. On the contrary, it would have been rather impossible for her to have not known Snape was the snoop. The idea that Snape could have heard the first couple lines, been apprehended by Aberforth and chucked from the building by the time the prophecy ended doesn’t work. So it just makes sense that Snape was still struggling with Aberforth when the Prophecy ended. It also makes sense that, as he was in the process of wrestling with Aberforth, that he didn’t hear the second part of the prophecy.

So Trelawney’s and Dumbledore’s accounts don’t contradict each other in the least; quite the opposite, they complement each other.