Hey Scott,
You don’t reduce threats when you do a swarm roll, the swarm roll reduces the number of obstacles in that room, which are different to threats.
You don’t need to roll to see what room your in - you do that during the setup when you construct the tavern. You simply reference the number on the dice that represents the room you’re currently in, which can inform what obstacles might be present.
If you check the “Introduce a Threat” section of the rulebook, you can see what effect each of the different threats do. For example, the rat-catcher stops you from being able to use the scurry stat.
Hope that helps, and thanks for playing!