Is there any definitive codification of sharia?
Nope. It depends on who's interpreting the Quran, Hadith and Sunna, all of which are used to determine law. The veracity of Hadith (stories of Mohammed's sayings) and Sunna (stories of Mohammed's deeds) are graded generally on how believable they are, how well they fit into the collector's view of what Islam should be, and how far back close to Mohammed they can be traced. So in making Shari'a, a Mulla gets to pick the stories that fit his point of view, then he gets to pick how he'll interpret those stories. Obviously, this leaves lots of room for things to get twisted.
Also, not all Muslims believe they are authoritative, and different Muslims accept different Hadith and Sunna. For example, Sunnis trust anything coming from Aisha, as they see her as good and wise, while Shiites despise her as the jealous, unfaithful usurper of authority.
It's kind of funny, because the Quran says others will come later to fabricate lies about Mohammed, and that can easily be taken to refer to Hadith and Sunna. The worst part is that their religion says the Quran is "complete, perfect and fully detailed," so why even the need for Hadith and Sunna? I see a simple need: for people to twist the religion to their point of view.