Only a Hogwarts professor, Dumbledore, and maybe Lupin could do that.
What is special about Lupin that he could pull it off? The professors would probably have security clearance. Dumbledore has security clearance and is incidentally freaking Dumbledore but Lupin is just some wizard. He’s fairly competent but not so much as, say, Mad-Eye. I suppose he has better than average Hogwarts-security knowledge due to his misspent youth...
The evidence against QQ is pretty strong;
H&C cast a memory charm on Hermione without triggering the wards.
Only a Hogwarts professor, Dumbledore, and maybe Lupin could do that.
H&C either wanted Hermione to be blamed for attempted murder, or wanted her to succeed.
That rules out everyone is isn’t willing to kill innocent children to advance their plots. Realistically, only QQ fits.
Whether Harry has enough information to figure it out is a different question.
What is special about Lupin that he could pull it off? The professors would probably have security clearance. Dumbledore has security clearance and is incidentally freaking Dumbledore but Lupin is just some wizard. He’s fairly competent but not so much as, say, Mad-Eye. I suppose he has better than average Hogwarts-security knowledge due to his misspent youth...
Lupin was brought in as a special instructor for the Patronus charm, thus might possibly have professor status.
Snape falls into the first category. That said, I agree that Quirrell is almost certainly the actual culprit.