A lesion would be red, raw, and very noticeable and probably have some sapro around the edges. Judging from the pic, Shigeru looks like he has the faintest dragonscale markings on his head and with dragons, a missing scale can look like a pothole because of the thick scaling. Since it seems healed over, most likely it IS marbling like you thought.
EDIT: Those fish are wild fish; pics are from fish and game, I think.
EDIT II: In columnaris, it's not so much lesions as it is "fuzzies." Those kind of ulcers like you see in that pic would typically be caused by mycobacteria or vibrio bacteria in the aquarium (positive diagnosis would have to be taken by a skin sample under a microscope to rule out anything else).
So the columnaris on his fins has receded a little which is great, but it's not really going away on his face yet. I have a feeling this is columnaris that has survived and developed a resistance to metro and it might be resisting the kanaplex. I saw a little bit of red on Shigeru's tail today, but the fin melting seems to have halted for the moment.
This is just to discuss it ahead of time, but in case Shigeru really doesn't poop before he hits critical mass, is it possible to fast him from medicated food? does that interrupt the kanaplex's effectiveness at all?
No, I think fasting him from medicated food would be okay. It's more important to not put anything in if nothing is coming out and he'll be getting meds from the water, as Kanaplex is absorbed really well by the gills. Let's see how the Kanaplex does after a full course. If the columnaris is still there, we can think about adding a second med.