Two points:

• Did anyone remember to take the "42" envelope, before returning Hermione's effects to her parents?
• Are Harry's and Hermione's parents allowed to talk to each other?

Has anyone else noticed that Quirrell knew James Potter?

"James Potter," said Professor Quirrell, his eyes narrowing. "The boy is not much like James Potter.

Hmm. How about having someone else die in Hermione's place?

I don't recall offhand if the death burst was recognizable as Hermione, but otherwise it seems doable. Dumbledore said he felt a student die and only realized it was Hermione once he saw her.

You'd need polyjuice for the visual appearance, and either Hermione's presence or a fake Patronus for past-Harry to follow. Hermione is unlikely to go along with the plan willingly sho she'd need to be tricked or incapacitated. Hard to tell which would be easier.

Given the last words, Hermione's doppelganger might need to be complicit with the plan. Easy to accomplish if it was Harry, but I think he's too utilitarian for that. He'd need someone loyal but expendable. Lesath would seem to fit the bill, but I wonder if he'd agree to literally die on Harry's command.

True.

Also, I assume if they saw Tom Riddle or Salazar Slytherin or some other name they consider utterly evil, they'd tell Dumbledore immediately, rather than dismiss it as a bug. Even if they've never heard of Pascal's Mugging, that's just the sane thing to do.

Quirrell can feel Harry's emotions. This can partially explain at least some of the cases when he unexpectedly realized what Harry was thinking (for instance, this probably gave him some information during their conversation about Parseltongue). It might be worthwhile to find all cases when they talked and Harry attempted to hide his emotions.

The second bit does not accurately distinguish between a frys-njner ragvgl naq n aba-frys-njner ragvgl gung whfg vf "cebtenzzrq" jvgu gur erfcbafr bs "lrf" gb "ner lbh frys njner?". Hayrff lbh ner vzcylvat gung fbzrubj nfxvat gur dhrfgvba jvyy znxr gur cnvagvat orpbzr frys-njner fbzrubj?

I think the simpler "conscious Horcrux inside Harry" theories are ruled out by the sorting hat: "I can tell you that there is definitely nothing like a ghost - mind, intelligence, memory, personality, or feelings - in your scar. Otherwise it would be participating in this conversation, being under my brim."

It is possible that the original, canon Harry has been completely replaced by Voldemort, or that the Horcrux has merged with Harry to form a new single personality. The sorting hat is also explicit that it does not remember previous students such as Tom Riddle as individuals. Therefore it would not notice if HJPEV were in fact Tom Riddle redux. However there's just one person in Harry, not two.

I was thinking it would work as effectively because of the fire weakness thing and Harry shouldn't be magically depleted yet. Also it would be a lot safer than transfiguring sulfuric acid. Although it's possible the troll was enchanted to make it fire resistant and that Harry frankly didn't care whether he was violating the rules of transfiguration, and merely was worried about speed, but I'm inclined to think incendio would be faster.

I assumed that it was sarcasm.

"So," Harry said, "you know those really simple Artificial Intelligence programs like ELIZA that are programmed to use words in syntactic English sentences only they don't contain any understanding of what the words mean?"

"Of course," said the witch. "I have a dozen of them in my trunk."

Did she mean that she had muggle computer programs? Or did she mean some magical artifacts that work in the same way, or was this just a simple misunderstanding?