B.Sc

Sorry, you cannot, you need at least 90 for CBSE/ISC or 85 for State Board to get into SXC.

The answer is solely depending on Subject. Xavier's is famous for Mathematics, Chemistry, Microbiology, Biotechnology and Economics, whereas, Presidency is famous for Statistics, Physics, Computer Science and biology (zoology and botany).

It varies year wise and board. Try to secure 95+ (CBSE/ISC) or 90+ (State Board) to ensure your admission.

Since you were preparing for ISI, I assume that you are good in Mathematics. If you want to get a strong mathematical background before pursuing Masters in Mathematics, I can recommend you following institutions to apply.
1. ISI
2. CMI
3. IISc and IISER (especially Pune is well known for mathematics)
4. IITs
5. St. Xavier's College, Kolkata
6. St. Stephen's College, Delhi
7. RKM Vidyamandira, Belur
8. RKM residential College, Narendrapur
9. Presidency University, Kolkata
10. Jadavpur University, Kolkata.

Yes, BHU offers microbiology as occasional hons which is equivalent to hons., so you can appear for any all India examination in microbiology for masters and no any university or college refuse.

Engineering will be helpful than M.Sc. You will find a lot of opportunities and practical experience in B.Tech rather than M.Sc but B.Tech is not a good option, you should go for M.Sc first then direct M.Tech in chemical engineering as M.Tech requires Master's degree or 4-year engineering programme. All the best.

The Maharaja Sayajirao University of Baroda in Gujarat and KSKV Kachchh University in Gujarat.

Go for MCA degree but before it learn basic programming languages like C, C+, Java and understand all the basics clearly in your mind. Participate in competitive programming in Hackerearth, hackerrank, codechef, etc.

Yes, you need to fulfill the admission criteria of Galgotia for minimum percentage i.e 50%.

No, B.Sc in CS course in BHU is not available. For all courses available, visit http://bhuonline.in/courses.html.