Science

No dear, this is not possible to do class 12th with Commerce, and then do B.Sc. So, you have to repeat class 12th with science stream, because both the streams are completely different from each other. And, B.Sc in physics is again a difficult one. But, if you are sure now to change your stream, then you must pursue class 12 with science again.
Yes, most importantly please think twice before taking any step, as it is about your whole life. Your single decision can change your entire life, so please make sure that you must be clear with what you are doing.

Your percentage is too low to take admission for this course.
For admission to the course, candidates need to have qualified 10+2 from a recognized educational Board with a minimum aggregate score of 50% with Physics, Chemistry, and Biology as main subjects.
If it happen recently, then you can go for improvement exam (every board gives a time period of minimum 2 years to improve your marks once).And continue your preparation for your respective career option simultaneously.

B.Sc with computer Science is one of the emerging course. Being a computer Science student, career is never going to end. There is always an open area for them. As a career option, you can join army, navy, IT sector, etc. Along with B.Sc + M.Sc, you have to do some certificate course and try to develop more programing skill. If you do all these things that I have mentioned above you will face no harness to get job. It suite in your relevant field.

No sorry. Dalmia has HR only in PGDM and not MS.

You should have Biology as main subject in class 12th and score 60% minimum for admission in any course.

Which category do you belong from?

Yes, you are eligible for M.Sc Zoology.