So recently I got stuck, but I figured out a way through this part of the dungeon.
How to find the key in the Central Line
Go Back To The Start Of The Area
Go back to the start of the dungeon and talk to a demon behind and to the right of the main entrance after getting past the first miniboss (the one with the old man).
Wait around for a demon to spawn here (Not the enemy demon in this screenshot)
This dialogue should play.