Accounting & Commerce

You can pursue MA, M.Com, MA, PhD, etc. Categorically, there are more options in each of these as per the field you choose.

It is company secretary and not computer secretary. It's taught by the Institute of Company Secretaries of India. Eligibility-
Foundation Programme -12 pass or equivalent
Executive Programme -Graduation
Professional Programme -Executive Programme.

Foundation Programme= Rs. 3,600
Executive Programme= RS. 7000
Professional Programme= Rs. 12000.

CA or CS are professional degrees which you can pursue right after you complete in 10+2. If you want her to study something along with CA or CS then she may opt for B.Com or BBA. CA or CS requires a student to put in a lot of efforts.

No, you can't, even if you take commerce.

Eligibility is having Maths in +2 and passing in the subject. Their cutoff for the respective year, entrance exam rank/minimum score set by the college (not sure about this as DU will be having an entrance for the first time in courses like B.Com/B.Com Hons).

You can change your stream. You can pursue BBA. You have to start off as a fresh candidate right from the beginning of the first year. You can also change your stream to any other subject. Make the correct decision regarding the stream, which you want to pursue with. Also, ask your friends from various streams to find out about different subjects and to increase your options.