B.Sc

Yes, you can get admission in St. Xavier's College, Mumbai with 81%.
Good luck!

Yes, you can pursue PGDCA from a deemed university. You can also prepare for CAT or other competitive exams.

Lab technician course is a far better option than SSC, and LDC because after having technical knowledge you can get a job in the field of education which is a noble job. Also, you can get good salary package too. Whereas, LDC has less scope as compared to lab technician course.

After B.Sc in mathematics, you can pursue M.Sc in mathematics. Following are some other course options which you can pursue after completing B.Sc in mathematics:
M.Sc in statistics.
M.Sc in industrial mathematics
B.Ed
M.Sc in mathematics with computer applications
M.Sc in statistical applications
M.Sc in applied mathematics
M.Sc in applied statistics.

It depends on what your priorities are. If you want to start your career as a teacher then go for B.Ed first. Then you will be eligible for TGT posts. But if you want to continue into research field then you should opt for an M.Sc and then try for a Ph.D. All the best for your future endeavors!

Hi Jeevan,
Ideally, you can, but please write to the universities before hand. The right to accept your application or reject it rests with the university administration. They will evaluate your certificates as per their norms.