Here is the location of the arcing, after cleaning most of the wax from the flyback HV coil.
The view in this picture is about 5/8 inch wide, with the darkened area being just over 1/4 inch long. My main concern would be whether the dark parts are now carbon or something else that I should attempt to remove, before applying some corona dope (or other substance such as HV-rated epoxy as Eric mentioned?), followed by the silicone.
Also, I doubt that I could remove "all" of the wax on the coil, especially after seeing discussions of vacuum-impregnating flyback coils. I would be concerned that the materials or methods to do so could be risky themselves. Is some remaining wax likely to doom the silicone coating integrity as a whole? I appreciate any suggestions here.