Science

It is not possible to pursue B.Com and B.Sc simultaneously. B.Sc is pure science and you must have strong concept of the subjects to pursue it. You can first go for B.Com, then you can invest your time in understanding the science subject, and then you can go for B.Sc.
It means a total of 7 years is required to complete the whole process. But, every college has some eligibility criteria, which you must fulfill to get admission for B.Com or B.Sc.
Best of luck.

You should contact the university. If you want good education then opt for Pune or Mumbai University.

You can go for:
1. B.Sc.
2. B.Tech.
3. Integrated B.Sc – B.Ed.
4. M.Tech.
5. Integrated M.Sc – Ph.D.
Below mentioned are the colleges known for mathematics:
1. Indian Statistical Institute - Delhi, Bangalore, Kolkata, Tezpur and Chennai.
2. The Institute of Mathematical Sciences – Chennai.
3. Tata Institute of Fundamental Research – Mumbai and Bangalore.
4. Indian Institute of Science – Bangalore.
5. Indian Institutes of Science Education and Research - Mohali, Pune, Thiruvananthapuram, Bhopal and Kolkata.

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.

If your 1st year admissions is through any legal authority, then contact them to change the university. If they have such rights, they should help you.

According for me, you should go for civil engineering.
Below mentioned are few reasons:
1. You could land up in a government job, if you crack some competition exams and would get experience on your way to the top.
2. You can chose civil engineering as a mains subject in IAS. Only, mechanical/ electronics apart from civil have this liberty.
3. There is no dearth of money and opportunities after the first few years of struggle.
Below mentioned re the best colleges:
1. UPES – Dehradun.
2. IITs.
3. BITS – Pilani.
4. College of Engineering – Pune.

It depends on your interest.
Below mentioned are some of the branches:
1. Dairy technology.
2. Food technology.
3. Biochemistry.
4. Bioinformatics.
5. Microbiology.
6. Genetics.
7. Clinical research.
Best of luck.