The "bright glow" is the rim of the shell where it's broken, the flat surface of it would be angled to a point where it would probably be slightly lighter than the rest of the shell. If you look at the other broken points, you'll notice that they all have a bright rim around them, unless I'm misunderstanding what your point. Even if I made that part darker due to the arm, it would still be lighter than the area of shell around it.
As for the mouth, I don't think removing the AA will work. In fact, I tried it and it looked... bad, like his mouth was painted on, but I tried something else and I like it.
EDIT- To help show what mean with the shell, it's meant to be something like this:
Yeah, the arm probably would have more impact on the shading over it, but... idk, I'm hesitant to add extra shading in that small area as I think it may make it harder to read.