To me, the Claymore body mystery revelation thing is just fine. I was guessing something more grotesque too but that doesn't mean my satisfaction has to base on the grotesque level.
I think the concept of 'injury-before-transformation' would work here. Like, the massive frontal cut was done prior to receiving youma power and therefore would not heal (healing is kinda like returning the body to its initial state, and the initial state is their body with the scar.) However for fully-awakened beings, their bodies are further tweaked/transformed, and the so-called 'initial states' of their bodies no longer apply since they get a different form. That's why there ain't scars on the Abyssal ones.
In any case, loving the reunion, it's going a little slow but hey what the heck, I'm loving this series!