Part that you need for context here
This was supposed to be the last part, until I noticed how long it was and decided to split it.
So. For this comic I am running with the headcanon that Nice can't get scars because he's perfect and that any he might have had from when they just started had vanished.
As you see, Wreck only has scars on his back. Which is what he means with accidents. Nice I imagine would always attack from the front and make sure to not actually hurt. But a body hitting the ground will get some damage. I'd think that most of the scars are from early on when they weren't high ranking and Wreck didn't really know his powers yet.
To answer the question why the crack stays. I think it's very lovely symbolism for trauma.
I read somewhere (I do not remember where tho, ALWAYS CITE YOUR SOURCES KIDS DON'T BE LIKE ME!) that we experience a lot of traumatic things in our lives. But only few actually show up as trauma. And it has to do with not being properly taken care of after the incident and feeling out of control. Which is why a majority of trauma comes from childhood.
So. If we run with this idea (which I do). Wreck can't leave any marks on Nice because Nice feels cared for and safe with him. So even if Wreck were to hurt him, it wouldn't last.
The fear cracks however come from all the trauma the industry, Shang De and everything else did to him, which I assume he decided to shoulder alone. Which fits the feeling out of control and not being taken care of after incident to a T.
I do hope this makes sense.
Nice be like: Bro shut up I'm trying to cope here. (Trying to spin his cracks into a narrative that's not terrible by the color reminding him of his safe place, Wreck)
And Wreck be like: Yeah no we need to address the actual problem here please.