B.Sc Maths

Please refer to the link provided below for details regarding admissions in Barkatullah University for a course in M.Sc. You can also download their brochure for further details.
https://www.shiksha.com/university/barkatullah-university-bhopal-25770/admission#admission

Please refer to the link provided below for details regarding admissions in B.Sc courses in Bundelkhand University. You can download their brochure which contains information regarding rankings and reviews, placements for the institute for the past few years etc.
https://www.shiksha.com/university/bundelkhand-university-bu-uttar-pradesh-other-25207

PG Courses after B.Sc:
B.Sc graduates have a wide variety of options available in terms of topics, subjects, & field. They have an option to pursue a post graduation degree in their respective field. Students can also join non-science master degree courses such as animation, management, journalism, computer technology, etc after the completion of B.Sc degree.
Just like academic courses, students who have a B.Sc degree also have excellent employment opportunities. Jobs for B.Sc graduates are not just restricted in the field of science but also they can explore others areas like management, engineering, law etc.

You have no chance. If you want to do BCA, then it may be possible.

You may or you may not. Every college has its different guidelines, so contact to your desired college.

Yes, IIT Jam is a good option, otherwise you can also give entrance for BHU for M.Sc Mathematics. BHU is also one of the best colleges of India.

Qualification
12th
Additional Details
Mandatory Subjects : Physics, Chemistry, Mathematics
Candidate must have secured minimum 65% of marks for General Category, 58.5% of marks for OBC Category, 48% of marks for SC and ST Category and 40% of marks for PWD Category in mandatory subjects.

Please refer to the link provide below for a list of top colleges which offer B.Sc courses. These ten colleges have been ranked the top 10 colleges in India which offer B.Sc. courses.
https://www.shiksha.com/science/articles/top-10-bsc-msc-colleges-in-india-blogId-13473

BBA is open for all branches students. You are eligible to seek admission in BBA based on your class 10+2 results. If you are about to complete your B.Sc degree, then it will be better for you to do MBA instead of BBA.

Yes, you can definitely pursue it with minimum 60% marks in class 12th.