It seems pretty clear to me that mutation does take place. We've all seen pictures of two headed turtles, etc. Is this the method by which the designer makes changes?
Your argument is one large, intricately woven straw-man argument — and it is a straw-man that I am constantly seeing on this forum, used by both creationists and Darwinians. That straw-man is how most of the people here are defining intelligent design. Intelligent design holds that certain features of the biological world are more adequately explained by an intelligence rather than a mindless process. This is the theory of intelligent design. Intelligent design does not define the designer, and as such your statement doesn't ID dictate that a child with a liver disease was put here on purpose by the great designer is irrelevant to the theory of intelligent design. Detecting the work of an intelligence in a biochemical system simply cannot tell us who the designer is. For all we know, the intelligent designer or designers are extinct by now. From the standpoint of detecting design in biochemical systems, the identity of the designer is simply irrelevant.