Just so you're aware, there are NPC specific Pressure Plates [maybe Yellow?] that only activate when NPCs interact with them. If you care enough, you could rig a simple Teleporter System to self-correct for this one instance, so the Traveling Merchant doesn't have to die, or until his A.I. gets "fixed"....but every time the traveling merchant comes to visit the NPCs there, he drowns. It's fine when he visits another town, though.