CBSE

Hi,
Yes, he is eligible for JEE mains and other exams but there is an rule by CBSE that if you are giving improvement exams then you are not allowed to use previous result anywhere for higher education so kindly consult CBSE for better details and cutoff her name from B.Sc.
I hope you understand.

Hi,
You need to learn, solve and revise the NCERT books thoroughly. You can try to solve PYQS previous year questions.
I hope this helps.

No, it is not necessary. You can choose any place in India for improvement exam center. But, you state quota will be considered where you have appeared for 12th boards for the first time.

You can get them from market book store. You can also check CBSE's website and mycbseguide.com.

Hi,
Both the exams have same syllabus and concepts. If you are good with NCERT books, then you don't have to worry about JEE Mains.

It depends on the school. Different schools have different criteria depend on the number of students and infrastructure available.

If you are clear with your basics and your concepts are rigid, then half of the thing is done. Now coming to increasing your efficiency, you can practice more previous year question papers and questions from a particular book.
Best of luck.

Your maths marks won't matter as long as you do good in your BBA and MBA entrance exam. Just prepare yourself with a good explanation for low marks in the interviews.