Hmm, interesting theories altogether, and maybe true as well.
However:
- The Polyjuice potion was mentioned as well because of Crabbe and Goyle being disguised as little girls to alert Malfoy in the Room of Requirement. So the reason from emphasizing the potion could as well have been this.
- Why is it that Dumledore needs to go underground to find the Horcruxes? He's a potent enough wizard to go out and about on his own, he doesn't have to fear anybody. No need to use Polyjuice potion for that.
- If he did use the Polyjuice potion, and it was someone else killed on that fateful night, then it surely wouldn't be Sirius Black. Remember what happened to him in Order of the Phoenix? He disappeared through one of those portals/veils whatever they were. They couldn't recover his body through that. I admit that Sirius' death was rather all of the sudden, but I think that's intentional, because for both the reader and Harry it would be a shock. On top of that, would Dumdledore keep Sirius from Harry? And why?
Malfoy and Potter teaming up would indeed be a good option. Good thinking!
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