I don't understand how Quirrell could possibly be behind the troll situation, given that he apparently didn't know where the troll would be and had to resort to extreme measures to get to the scene quickly, instead of conveniently waiting just nearby.

One level above.

Edit: Actually I take that back, because that means all evidence would point to Quirrel no matter what. This is a fully general argument for Quirrel being behind everything ever.

1) Trolls move around. 2) Quirrel had to be in the great hall with everyone else to allay suspicion 3) I don't think he planned on Harry confronting the troll. He only started going to where the troll was when Harry confronted it. He only knew where the troll was because he knew where Harry was. 4) If he planned on Hermione being killed (as opposed to the troll being a distraction for getting the Marauder's Map or some other scheme, as in Canon), it's in his interested to "search" for the troll far enough away that it has time to chase down and kill her before he can get to the scene, because not arriving in time is less suspicious than "failing" to save her.