The law of Shari'ah regarding inheritance, wasiyyats (bequests) and Islamic wills

Q:

1. What are the guidelines of Shari'ah one needs to keep in mind when distributing the inheritance of the deceased and is it compulsory for one to prepare an Islamic will in his lifetime?

2. If one did not prepare an Islamic will, then how will his inheritance be distributed among his heirs after his demise?

3. What should be included in the Islamic will?

4. Is it permissible for a person to bequeath wealth for an heir or a non-heir upon his demise?

5. How will the estate be distributed in the case where one has debts and one has also made a bequest for certain people? 

6. What types of bequests are permissible and impermissible in Shari’ah?

Making wudhu with nail polish

Q: Why is wudhu not valid if someone wears nail polish? Wudhu is a matter of cleanliness, so if anyone wears it after cleaning their nails then the dirt will only touch the nail polish. When performing wudhu, the nail polish will be washed. So why should the water should reach the nail?