Medically speaking, it wasn't bad enough to be noticed. The scars were mostly on the surface (i.e. not breaking any internal organs). Although yes, it was severe.
Derek's father has his own story which unfortunately I cannot cover in this story. I might end up writing it some day.
As a silver lining, no, he was not screwing around on Mrs. H. If anything, his lover for her remains the only thing whole in him. But again, it's something I will not discuss in this story.
Perhaps in a sequel...
And yes, Derek's "perfection" ramped up when he went a personality change (which slowly toned down as more of his buried feelings returned).