I was just short an hour to report who is most likely the demonologist!
So, I'll try to explain what conclusions I've come to:
Clue 1 "Rope": We've already discussed who it might belong to. I don't see any tricks here. Amarant, Arandor, who else?
Clue 2 "Red Stone": I strongly urge you to pay attention to Comrade Amarant's signature. In his userbar, you can see the heroine from the anime series Rozen Maiden. Her name is Suiseiseki. As you can read on Wikipedia, it translates from Japanese as "jade." Jade can be red, although it's a rather rare color for this mineral.
Now that you know this, read Amarant's posts. He's definitely up to something. He was the one who noticed the rope with Arandor, and he was the one who tried to frame Mold as the demonologist (I also tried, but that's in the past :o). And he didn't kill anyone that night. Probably so that we would all forget about the stone and the rope. So, are there any arguments against this?
Execute Amarant!
Bow down and hear our cry,
Our earthly rulers falter,
Our people drift and die,
The walls of gold entomb us,
The swords of scorn divide,
Take not thy thunder from us,
Take away our pride.
G.K. Chesterton: English Hymnal