Exam

If you get admission based on your 12th you may take admission in BA. Also, you may appear for maths reexamination as it will be helpful to you in future.

Hi Sarthak,
There is no negative marking in IELTS. So you can answer everything.

To prepare CAT and MAT, you can take help from any tutor, you can take guidance from mentor or can fix any coaching class.
You can study from online tutoring websites also. There are many websites where you can study online from teachers, sitting at your home.

If you get good percentile in CAT, maybe you won't have a problem in getting admission. Just work hard and study well.
If you need help for your CAT preparations, you can take classes from online tutoring websites also.

Only if you have completed your graduation from a recognized university then you may apply at SCDL for the programs offered by SCDL.

Have you appeared for NATA 2017?

Definitely you should go for MBA. I am also doing B.Tech in CSE and will do MBA further. Firstly, you have to give CAT, MAT or GMAT exams through which you can get admission in IIM. I will suggest you to do MBA only from IIM or any reputed private colleges like LPU, BITS, etc.