So what mechanism do you suggest Voldemort used, in light of the above quote?

I read "another" as someone besides Quirrell. I don't see how that disagrees with, say, V reading Flamel's mind and then Bellatrix AKing F, or V reading F's mind and then commanding F to kill themself, or so on.

The most obvious interpretation is that V just sent B to kill F, or Owled F a hand grenade, or so on. But I don't see why V would prefer that option, especially given that even in the world where V has F's lore, V would want Dumbledore (at least) to believe that F's lore is gone.

Nicholas Flamel is dead, at least according to Dumbledore. (Or tucked away for later secret extraction?)

I.. also just realized that "Flamel" can't possibly be dead. The rite Voldemort used on Hermione was not one of his own devising, but a piece of lore well known enough to have a usual result. "Flamel" had the stone of permanency for either 600 years, or much longer than that. And has more lore than Voldemort.

Had more lore than Voldemort. Legilimency is fun.

Okay, Harry is really overdoing it here. It would have been much safer to pretend utter ignorance of everything, or at least to limit his reaction to falling over. The scene as set will cause sufficient theorizing without trying to force a particular narrative.

On a meta level: Getting this scene from a bystander means they are not in the mirror. So that's that.

I.. also just realized that "Flamel" can't possibly be dead. The rite Voldemort used on Hermione was not one of his own devising, but a piece of lore well known enough to have a usual result. "Flamel" had the stone of permanency for either 600 years, or much longer than that. And has more lore than Voldemort.

The best creature to assume the essence of from a defensive standpoint isn't a troll or a unicorn. Tough, heck, if there is no downside to just stacking things, maybe she did. The creature in the potterverse with the most absolute defense is the phoenix. Fiendfire? Firetravel away. AK? Respawn and laugh. So Dumbledore may have seen Flamel die, but that means absolutely nothing.

.. depending how the sacrifice works, this might not even hurt the phoenix you are using! Well, permanently anyway.

One obvious problem is that I doubt Salazar would murder students on request, but heck, Voldemort could have set Myrtle up without his consent.

In canon, at least, the Basilisk is all into murdering students, suggesting victims and egging on the Heir. Indeed, it is the Basilisk's murderous mutterings in Parseltongue that make Harry aware of it.

Wait, are you suggesting that Slytherin's Basilisk is Salazar Slytherin, in Animagus form? (Edit: spelling.)

this is highly unusual for an obliviation

That's the resonance effect from casting a spell on another Tom Riddle, I think. (Was it also there for the transfiguration? Why not?)

1) (Harry tells Voldemort his death could hijack the horcrux network) doesn't seem unlikely at all. Both hints from within the story (the Marauder map) and on the meta level ("Riddles and Answers") suggest an unprecedent congruence of identity, at least in the sense of magical artifacts (the map) being unable to tell the difference.

I did not post it since strictly speaking Harry should keep quiet about it.Losing the challenge of not dying (learned to lose), but increasing his chances of winning the war. Immediately even: Since the new horcrux system enables ghost travel, Harry could just try and overwrite / take possession of Voldemort body. Either it works and he wins, or it doesn't and the magic resonance kills ... well, kills only Voldemort, since Harry at that point would be the undead spirit.

That solution occurred to me as I was reading the challenge, and I was puzzled that on my (admittedly cursory) reading of a bunch of solutions, I did not find any exactly resembling it. Either the approach is deeply flawed and I don't see it, or everyone else is taking this as literary as I did and holding off on proposing it (since it may not be precisely the teacher's password as worded in the challenge), or something else.

There’s one problem with transfiguring toxic gases: It is unsatisfying from a story perspective, and it would need to kill instantly —

There’s two problems with transfiguring toxic gases: It is unsatisfying from a story perspective, and it would need to kill instantly (since otherwise a coughing Death Eater might still be able to use non-verbal spells to incapacitate Harry), and it would need to hit all —

There’s three problems with transfiguring toxic gases: It is unsatisfying from a story perspective, and it would need to kill instantly (since otherwise a coughing Death Eater might still be able to use non-verbal spells to incapacitate Harry), and it would need to hit all Death Eaters at exactly the same instant (otherwise one DE could still get in a shot, when he sees other DEs dying.)

He would tell he can do it, but not necessarily how it works. Of course knowing that problem is solvable facilitates solution, but since we know the solution, we also know it would take time for Voldemort to find and use it.

So it is buffing, but with a time delay. That's why I think it's the simplest solution. Quirrelmort did start reading a book on physics, but is certainly far from understanding it deeply enough to do partial transfiguration.

This move would simply buy Harry time. It won't solve the problem of Voldemort threatening the world, but will keep Harry alive, which is the objective of this quest.

There is tonnes of things Harry could infer and then tell him

OK, then use that to buy Harry's own life. My idea was more about Harry buying his own life than telling about partial transfiguration.