B.Com

Yes, you can pursue MA in Mass Communication. It is the course which gives you chance to explore at every step and provides you many opportunities.

Yes, you can do 3rd year as regular.

Yes, you can submit an affidavit of the same.

You can write CAT exam in November XAT on Jan 6
Symbiosis PUNE, IIM, Jain University, and Christchurch University, Bangalore.

No, there are no shifts; only regular college timings. You need to score at least 50% marks in aggregate in order to be eligible for admission.

U need to secure above 80% to secure your seat in Delhi university as the cutoffs are high n there is no entrance exam for B.com u will be selected on merit basis and there some seats for reservation for categories like SC, ST etc so if u are in those category the required percentage will be less otherwise u need to score above 80% wishing you all the luck with hope u get selected for Delhi university good luck

Yes, you are eligible for the admission in M.Com.

Yes, definitely, it will be beneficial for you. Although the things will be slightly tough, but if you're sincere you will be easily able to both of them. There are many people in my knowledge who are pursuing both the courses simultaneously. But CS with MBA certificate will definitely be as an advantage in the corporate world. So, I suggest that after completing CS you should do MBA.
Good Luck!