1. Natural grading is based on points - points are only limited by your admin settings. You can have items worth whatever you want.
2. While, Natural will adjust the weights accordingly, you can override all or any weights along the way.
3. Natural course totals will be determined in the following way - total points attained divided by total points attempted. If an item has not been attempted yet, it will not be included in either side of the equation (unless you select that option in your gradebook setup - aggregate only non-empty grades). This provides the student with a running percentage in the class but does not infer that they have passed or not passed the course. You could use completion settings for that...If you want a running total, you can have Natural just show points, aggregate the empty grades and then the students will see how far they have to go. As a student, I always liked being able to slack off at the end of the semester. I figure if they have worked hard enough to make that many points at the beginning, they should get a break at the end but that does not work if you want them working consistently throughout the course!
The biggest obstacle I see is that you want a subcategory to carry 10% of the total grade instead of the category (that it belongs to) grade which is not really how subcategories are supposed to work. I suggest making your Subcategory a regular category and that would solve that problem. To keep management happy, you could have Category C - Exams and Category D - Exams (Final Exams) or something similar using the names of the categories to link them but keeping your math correct.