Quirrell storming into the trial when the majority of the audience believes him to be the one behind everything sounds quite like this story's style.

The trouble with this theory is that the arc is confirmed to last until chapter 84, and Quirrell being suddenly released from custody would be far too short of a resolution.

It is surprising that Quirrell would accidentally reveal himself as an impostor during interrogation; so, perhaps the Quirrell currently in custody is an impostor--meaning that he is not the Quirrell currently teaching at Hogwarts. If so, the imposter is there to give Quirrell time to do something else. He may be attempting to prove Hermione's innocence (even if he is to blame for the current situation), or he may also be after the Philosopher's stone.