Why did no one suspect Quirrell after Halloween?

Question

This is kind of a ‘sequel’ from this old question where it ended up as a tangent.

Professor Quirrell spends most of Harry Potter and the Philosopher’s Stone pretending to be a nervous, frightened man who faints at the sight of his own shadow. This is particularly clear in the scene that takes place on Hallowe’en:

Harry was just helping himself to a jacket potato when Professor Quirrell came sprinting into the Hall, his turban askew and terror on his face. Everyone stared as he reached Professor Dumbledore’s chair, slumped against the table and gasped, ‘Troll – in the dungeons – thought you ought to know.’

He then sank to the floor in a dead faint.

^{Chapter 10: Hallowe’en (p. 127, Bloomsbury paperback)}

Towards the end of the school year (and the book), we find out that this is pretense, and that Quirrell himself let the troll in. As he says, talking about the troll that constituted his part in the protective measurements guarding the Stone underneath Hogwarts:

I have a special gift with trolls – you must have seen what I did to the one in the chamber back there?

^{Chapter 17: The Man with Two Faces (p. 210)}

Now, we know that the Stone has been kept at Hogwarts ever since Hagrid took it out of Gringotts before the school year even began, and it seems at least highly likely—though it’s never stated outright—that the protective measurements various teachers provided to keep the Stone secret were placed around the same time.

In other words, we have Professor Quirrell providing Dumbledore with a fully-grown troll as a protective measure some time during August of 1991, managing to both catch it, neutralise it, bring it down into the dungeon-like underground place beneath Hogwarts, and set it up there.

Then, barely two months later, on 31 October (Hallowe’en), we have Professor Quirrell appear in the Great Hall, so scared by the mere presence of a troll in the school dungeons that he actually faints.

Based on Hagrid’s enumeration of which teachers provided protection for the Stone, it seems reasonable to assume that the teachers (at least the ones who helped with the protection) were aware of who did what for the protection. Certainly Dumbledore himself must have been. So at least Dumbledore, and probably at least some of the teachers, already knew that Quirrell had a certain flair for dealing with trolls.

So why did no one seem to find it odd that Quirrell was suddenly fainting at the mere sight of a troll? (Except Snape, of course, but he already suspected Quirrell before this.)

Or, to put the question on its head: Why did they all believe in Quirrell’s jittery act when they knew how he could handle a troll? (Except possibly Snape, if this discrepancy is indeed what led him to suspect Quirrell to begin with.)

Answer 1

I don't think Dumbledore trusted Quirrell. But it is not clear what made him doubt Quirrell's loyalty to the school.

Dumbledore turned a page, and said, without looking up, “Keep an eye on Quirrell, won’t you?”

_{^{Harry Potter and the Deathly Hallows, Chapter 33, The Prince's Tale}}

Snape kept watching Quirrell on Dumbledore's orders.

“Certainly. I have a special gift with trolls — you must have seen what I did to the one in the chamber back there? Unfortunately, while everyone else was running around looking for it, Snape, who already suspected me, went straight to the third floor to head me off — and not only did my troll fail to beat you to death, that three-headed dog didn’t even manage to bite Snape’s leg off properly.

_{^{Harry Potter and the Sorcerer's Stone, Chapter 17, The Man With Two Faces}}

It's just that Quirrell might not have know that Dumbledore had asked Snape to watch out.

Answer 2

The teachers wouldn't suspect a theoretical wizard would be bold enough to attempt all of the gauntlet. He is very well known for his book smarts not his bravery. As with fluffy I'm sure he had help placing a huge troll in the School without notice. So it is the old saying the devil you don't know in the end.

Answer 3

First imagine a dragon tamer. They are hatched and monitored with great caution. They are caught with difficulty. But once they are secured, they can appoint any weak minded person to do some maintenance work around the dragon cage.

similarly, Quirrell supplies a troll for protection of the stone. It could be because he has some connections from his previous works when he dealt with trolls to write a book. But doesn't mean he is not shit scared of them. Doesnt mean he wants to encounter one in a secluded corridor where it could smash him with one swipe.

Hence, he could provide troll to Dumbledore and still be out of suspicion for fearing them. A zoo keeper can sell a lion to a person and yet be the first one to run in the opposite direction when the lion gets free because he has seen its violence up close.

BTW did Dumbledore ever trust Quirrell?? I don't think so!