I woke up in the middle of the night with an idea that could still be true, but wasn't needed in Chapter 114-115. It had bothered me that the Hermione toe-ring wasn't detected. In fact, it was explicitly checked for magic and it was discovered that it was portkey magic rather than transfiguration magic.

Overnight, I had the idea that perhaps Harry put his portkey toering on Hermione's body (though obviously not her toe) before he transfigured her. I have no idea how such things work, but I suppose its possible that the portkey magic would still be detectible when transfigured and would even mask out the transfiguration magic.

Quirrel-supporters: don't worry, I'm sure Bellatrix will show up to save the day.

Ch. 108

"What did you do with Bellatrix once she was out?"

"Ssent her to a peaceful place to recover sstrength," Professor Quirrell said. A cold smile. "I had a use remaining for her, or rather a certain portion of her, and on my future plans I shall not answer questions."

Ch. 112

For a second Harry's mind couldn't process what he was seeing, and then he saw that Voldemort was holding a human arm, severed near the shoulder; it seemed too thin, that arm.

The Dark Lord pressed his wand to the flesh above the severed arm's elbow, and the fingers twitched, twitched like they were alive; by dim moonlight Harry saw a darker mark appear on that flesh, just above the elbow.

Is everyone else making this interpretation?

Beneath the moonlight glints a tiny fragment of silver, a fraction of a line...

(black robes, falling)

...blood spills out in litres, and someone screams a word.

Prediction: This is the scene where that happes, and the "fraction of a line" is partial transfiguration used as a cutting weapon.

I remark that

• it has recently been pointed out that Harry's Patronus v2.0 is powered by his life as well as his magic and that this (at least according to Voldemort, so obviously it's true) makes it more powerful than it could have been if powered by just his magic
• even the small fraction of his life he was able to give up on the spur of the moment was enough to restore Hermione's life and magic, which even Voldemort was unable to do on his own
• in canon, central to Harry's ultimate victory is his willingness to die

and suggest that if your prediction is correct, what powers his absurdly powerful spell may be the sacrifice of the whole of his life and magic.

(Hmm. The power of a potion in HPMOR is determined by what went into making its ingredients, a curious and probably important discovery that hasn't been applied yet for anything other than winning playfights. What went into making Harry was, among other things, the power and ingenuity -- and in some sense even the life -- of Lord Voldemort. Maaaaybe.)

Hypothesis: The mirror (or the whole room) connects universes. More specifically, there is only one mirror, stable in the multiverse (quantum, mathematical, magical or whatever) of compatible universes. A compatible universe is such an universe where the mirror exists, i.e. most probably the one that did not branch off before the end of Atlantis (or boltzmanned into existence a moment ago).

Looking into the reflection, your (magical) brain picks the image from a different universe, the one that matches your CEV most closely.

Nothing can hurt the mirror, unless the measure of the universes the action is performed in is a "significant portion" of the multiverse - otherwise it just dissipates.

The map shows gibberish, because, well, there is everybody and nobody in the last room...

Am I the only one who thinks it's not realistic that an eleven-year-old in the 1990s hasn't had any dirty thoughts?

If the laws of reality are simulated, then they must be computable. A giant switch statement isn't going to let you figure out how to make time travel consistent. They couldn't easily check every possibility and see if it's consistent. Even if they did, that would mean they're simulating all of them, including the inconsistent ones, and there'd be no reason for Harry to find himself in a consistent one.

Continuing on Izienwinter's reasoning:

Harry and Voldemort sprung the trap, they are now inside the mirror, Dumbledoor is outside.

It is now an AI box experiment, with Dumbledoor as Gatekeeper and Harry as the AI wanting out!

In chapter 25 the Weasley twins discuss the map

" How's it doing?" said Fred in a low voice. "Still on the fritz," said George. "Both, or -" "Intermittent one fixed itself again. Other one's same as ever."

I now think this refers to warm!Harry showing up as HJPEV and his dark side as Tom M. Riddle. If so, it's less probable the map is being manipulated.