CMAT

Minimum qualification required for MBA is graduation or equivalent degree. There are many institutes which provide MBA as well as PGDM courses, if you want to GET it from a top notch institute you need to have good scores in competitive exams too. As you are in 12th take your time, try to know more about competitive exams and institutes providing same courses. Fix your budget and then try to shortlist some colleges or take help of Shiksha. Com
At present IIM Ahmedabad and Bangalore are considered best for PGDM.

Yes, you will get the admission on the basis of your CMAT score but the cut-offs through CMAT is usually high as you will get the AI (All India) Seat in the process for CMAT score which is less compare to the state seats.

Better than just selecting only one college, I would like to suggest you appear for CAT, MAT and CMAT. This will open the doors of many other good institutes for you.

For general category, cut-off percentile revolves around 94 - 95 percentile. So, you have very less chances to get admission in BK.

Your admission depends on your category.
Cut off percentile for general category of last year was 94 percentile and for OBC category was 76 percentile. So, you can predict your chances accordingly.

4 institutes accepting CMAT scores-
Auro University
S R Luthra Institute of Management
UKA Tarsadia University
C. K. Pithwalla Institute of Management
For more information about colleges check the below link-
https://www.shiksha.com/mba/colleges/mba-colleges-accepting-cmat-Surat.

Yes, CMAT Score is accepted but you have to apply as a All India Candidate quota through DTE Process. And at this %ile you will easily get PUMBA.

Yes, it is valid but the thing is that in which college, you want to take admission? They will see that your marks is above eligibility criteria or not.

Apart from CAT and CMAT there are other college specific exams like SNAP, NMAT, etc. You may give it a shot. Note - all exams for admission in this year are over. Try for next year.