Islam my have their schisms, but they do not have rival scriptural authorities.You already put a lot of work into this post, I hope your further effort pays off somewhat better. I certainly would have been easier to leave off after your first sentence or so, and counseled patience while you dug out what others have spenttheir lives finding.
If as you say the Hadith's and Sharia (not to mention the etc.) are to be considered as "the scriptures of Islam" shouldn't we be considering the Apocrypha and the Old Testament as Christian scriptures. All three of my Bibles have the earlier testament that recounts the Prophets who foretold the Messiah, and the laws He talked about. Catholic Bibles are often printed complete with both the Old and with the Apocrypha. Not that we want to distinguish Prod and Papist strains of Christianity, but Islam likewise has had its share of schisms and rival scriptural authorities.
Surely if the history of religions has any one thing we should learn from our study, it is the gravely dangerous consequence of saying this and only this is the One True Word. Good luck, I doubt you'll find that certainty.
The Binding force of Islam is the Prophet Mohammad. As he is considered the mouth of Allah, and to question his word is to question that of Allah himself.
As such, whenever a line in scripture, regardless of the scriptural source, is attributed to the Prophet Mohammad it is beyond question. And there is nothing else in any other Islamic scripture that contradicts this basic edict.
Mohammad made sure of that when he created the religion.
So when such scripture attributed to the Prophet himself says bluntly to kill a person, That is the word of Allah. And it is not open to interpretation.
Now, some Muslim schisms may choose to directly ignore that edict from Allah, but they are knowingly abandoning a part of their religious teachings.
The Christian religion differs in the fact that the Christian prophet Jesus, who is generally agreed upon to speak for their God, directly contradicts the more violent and aggressive scriptures attributed to their God in the Old Testament.
Further The Prophet Jesus was never attributed to ordering death and punishment from his followers.
The Old Testament is included as a matter of context, as it established the history and the basic theological creation of the one God concept that they still agreed upon. After all, they were Jews to begin with.
But the fact that the Prophet Jesus was given the authority of their God himself, he could effectively overrule any aspect of the Old Testament. Which was done extensively.
And all this overruling was mostly centered in putting a more pacifistic spin on the more violent and intolerant aspects of it.
Something that is long overdue in the Islamic Religion.