B.Com

Go for bank coaching immediately after the start of 3rd year. TIME is one among the best for that. Rigorous mock test helps you gain more knowledge and you can easily crack the exam.

Assuming you are already 21 years in age at the time of completing B.Com and are a general category student you have surpassed the maximum age to appear for JEE for IHM so you will have to settle for private colleges. Most of them will decide according to your 12th or equivalent marks and GD or personal interviews, etc. depending on the private college. Keep in mind the college's reputation does come into play in the initial years.

Yes, you can do B.Com with computer science as a subject in 12th.

You need to write MAT exam for that. You can sit for MAT 4 times a year in the months of May, September, December and February. Please go to the MAT official website for the detailed information about MAT. Once the scores are out, you can apply to different colleges according to your score.