If you are a devout Muslim you are subject to many laws given by the one true God.
Not really. Most of the shariah laws come from hadith and the analogy and consensus of the scholars (qiyas and ijma). This is what you are taught in very basic level fiqh classes. The basis for the law may be in the quran or the sunnah, but the law itself was derived by some man. I can see where a simple Muslim who hasn't studied, though, would think that the shariah is from god, but this is not taught at all by the ulamaa.
These are infallible, universal and eternal laws. They may seem reasonable to the believer, but with some scrutiny they tend not to be so universal after all. I can do a few examples.
What Big Al and his messenger deemed halal and haram is halal and haram for all time, or the rules laid out are the rules for all time. But shariah is not static. Even the "ijtihad is closed" sunni ulamaa come up with new rules or do away with old ones all the time. It may not be popular or widespread, but it happens, and every madhab has a wide range of rulings, from majority rulings to minority ones that one may follow.
Imagine an orphan girl, raised as a Muslim until she goes to live with a very, evil man. After several years she has forgotten about her childhood teachings, and all she associates Islam with is the evil man she lives with. Because of this she renounces Islam. She is technically an unbeliever, but for all the wrong reasons. Will she go to the Hellfires? If not, is there any verse in the Quran or elsewhere that says she will not be punished?
I would have to see the verse in the quran to comment on it, but if familiar with classical Islamic thought, then you will know that this is an area of ikhtilaf, or difference in opinion, among the scholars. For example, Abu Hamid al Ghazali says that people who do not have the true teachings of Islam shown to them will not be held responsible for their lack of belief on the day of judgment... So, in theory, if all they knew of Islam was Osama or an abusive father, then they wouldn't be held accountable by god for not becoming Muslim. Ibn Arabi and Ibn Taymiya, as another example, both said that the hadith qudsi that "my mercy overtakes my wrath" means that eventually, the fires of hell will be extinguished and all will enter jennah - including disbelievers, although they may be at the lowest level of it. In other words, it's all very uncertain, and that is what Islam is about - putting uncertainty about one's post-death fate into the heart and minds of the believers and controlling them through that fear.
What if a man dies and leaves two children: a wealthy son and a blind, disabled daughter who is not married, and has no one to provide for her. It seems rather unreasonable that the son should get twice that of the daughter, who really needs it. What if the son is a total jerk and doesn't even like his sister, and doesn't want to help her. Doesn't he then have Allah's blessing to keep the bigger share?
My dear, this happens *every single day* in the Muslim world - and she need not be blind and disabled. I reckon some of us have female relatives that this happened to at some point. If it happens, you know what people say? "It is a test from allah for her," or "Well, he's the man, he does deserve more / it all." (Do you think that Muslims really even follow these most basic rules?) or "She must have done something shameful we don't know about... clearly she is a bad woman for her own family to do that to her." or "allah will punish him in the next life for what he's doing... what's for dinner?" (And if not her brother, the belief would be that it is incumbent upon other male relatives to care for her, and if they don't, the sin is on them as well -- but you know, in the afterlife... not here and now). In theory, according to shariah, the daughter can take the case to the court and the court would award her her portion. In reality, she may not even get as far as filing the case in court. Because she's poor, she's disabled, she's a woman, she's under pressure from other family members to let it go, she gives up and gives in, whatever.
If the parents fear that their son will do this, they can leave a *bequest* to the daughter. The estate is then divided on the quranic inheritance shares *after* the bequests are given. In this way, parents who only have one child - a girl - can ensure that their entire estate goes to her and not their own brothers, etc. Of course, the quran being incomplete and vague on most things, this is yet another ruling that was put in by the ulamaa when they confronted real life complications and variety.