I posit a roughly 70-90% chance that Quirrel was behind the troll. (large range; I could probably boost the upper end to 95% with "quirrel got X to unleash the troll". )

why not 95%+? well. 1. Quirrel never actually admits he was behind the troll to us readers. 2. Weasely memory: tampered with.
Two OTHER characters have been in a good position to do this: Dumbledore and Aberforth.
3. Snape got his wildcard status restored. And...I never really understood WHICH results of the whole SPHEW fiasco he actually liked (even if the whole thing was "according to plan", that doesn't mean he had to like ALL the results-he could have been constrained by maintaining status quo with dumbledore...). With Snape declared as at LEAST a level 2 player...he could have faked relaxing after harry's revelation. or it could have been genuine. 4. Sprout. -at about 1.25-2.5%, formerly 5%. "last suspect" updates are weird.
5. Lucius Malfoy: he has the motive. pretty slim slice of the pie, though, as he doesn't really have access.