1) does the Qu'ran outline Allah's reasons for his commands, or do you simply belief he is sovereign and has no need to explain anything to his followers?
2) is predestination (God set everything up to happen that way, he controls all) a core belief in Islam?
3) I hope this doesn't sound offensive, but I heard someone say that Muslim men can marry any monotheist (people of the Book) and muslim women can only marry the devout muslim men, as to keep them pure. is this true, or was I misinformed?
You have asked some tough questions there and due to my lack of qualification, I may not be able to answer them correctly.
1. We have been informed about the benefits of some actions and not others the rest were filled in by the Prophet Muhammad (peace and blessings be upon him)
For example praying five times a day, it may seem pointless but once you have performed over a period of time, you become a different person. More calm, more sound the heart is in a state of tranquility.
Paying charity this takes the greediness out of your money, you give and you feel much better about yourself and you don't feel so greedy.
2. We believe that God has given us free will but there are certain things we cannot avoid. We also know whatever he does for us this is the best.
3. Its partly true Muslim man can marry from the people of the book. The reason why a woman cannot is because the non-Muslim man does not believe what she does thus he is under no obligation to provide food, clothing or housing. Unlike the Muslim man who has to provide these things to his wife.