Ron's disguise in the book 7 Gringrotts break-in was a transfiguration (as contrasted to the polyjuice used by Hermoine), and explicitly removed by Thief's Downfall. It's not clear that Rational!Harry's transfiguration operates by the same rules as conventional ones, and could have been stored in some way to protect against exposure, but this provides both narrative and practical evidence against the theory.

I expect that point of the pen is to demonstrate to Lord Malfoy that a) Harry is a man of his word, and thus his words are worth examining, and b) Harry is a man of /exactly/ his word, and thus worth respecting.

In canon, Thief's Downfall undoes transfigurations.

"Effective" is not the same as "actual." Quirrel wasn't a horcrux in the sense that Harry or Nagini were horcruxes, even with what she's saying there. She just meant to say that Quirrel was like a horcrux. No ritual was done to make him into a horcrux.

????

My trouble with this theory is not that the dirt is lethal - after all, Harry has already decided Lucius's life is forfeit the moment he ceases to cooperate and that could happen at any time, contract or not contract, signing just means Lucius doesn't die soon - but rather that it seems like an extremely risky booby trap which could blow up (literally) the moment someone casts a finite-incantatem (which could happen anywhere by anyone for any reason), and it's actually more than a little suspicious if the ultra-high-security Gringotts meeting room where everyone is supposedly disarmed didn't involve a precaution like eliminating any transfigurations...

I grant 1), of course. But wizards have shields that ought to be proof against handguns. My question was asked in response to the line "guns that will reliably break a wizard's shields".

Yes, but "The Defense Professor" and "anyone else who can rig the wards" shouldn't have the same probability in his mind. What with all the rest of Quirrell's strange behavior and the curse on the position, "The Defense Professor" should be allotted a massive probability, with a comparatively smaller piece left for "whoever else has the ability to do this." He should be suspect number one by far.

We could just run through the whole list of Things I Won't Work With.

Muggles have never agreed not to value gold, even though vastly more money exists in non-gold-backed currencies today. Which is why those gold vaults still exist.

On the other hand, the main reason Muggles value gold is for jewelry, and it's more likely that wizards use magic to substitute.

But, again, the fact is that Griphook agrees to convert gold into Galleons at only 5% overhead. So even if a wizard only values Galleons, he'll want to acquire gold and give it to Griphook.

You left out "The wards explicitly say that the Defense Professor killed Hermione."

I mean, it is quite literally spelled out. I'm completely baffled at how both readers and characters are not promoting Quirrell to the top of the suspects list by a mile.