Some thoughts...

When reading through the first time, it did seem really obvious that Quirrell was an improved, much more rational version of Voldemort; so blatantly obvious that it made me think if it was a clear red herring. (In the same way that Snape is the canon red herring.) I wondered if Eliezer had reversed things, so that Snape is the real villain and Quirrell the real good guy...

However on re-reading, my prime suspect is now Professor Sprout (Chapter 13):

Maybe Professor Sprout was the Game Controller - the Head of House Hufflepuff would be the last person anyone would suspect, which ought to put her near the top of Harry's list. He'd read one or two mystery novels, too