Another night mission has brought in another suspect, possibly working in tandem with the previous suspect, and possibly many more.
I went and searched with my flashlight where the most recent damage was, where I had found the millipede, and found nothing. Good. I worked backwards to previous victims and noticed the back half of this guy protruding from underneath a semp's lower leaves. How I managed to spot him is a bit of a miracle, but he was in perfect position to munch away on low-lying stolons or leaves.
I believe this is a Cranefly larva. I see these from time to time in soil that I'm digging, but I've never seen one topside in action. It is not a cutworm as it has no legs or eyes. And Google images leaves me with little doubt of what this guy's true identity.