Commerce

Mathematics in 11th commerce is not much troublesome. Be it ICSE, State Board or CBSE.
I have studied all these books & hence I can tell you the level of difficulty is mediocre.
Mathematics is a subject that you can't mug up at last moment, So develop a habit of studying whole year if you are going to

No, you can't as science subjects are must to get admission in B.Tech.

The commerce courses available in Aligarh Muslim University are mentioned in the list below:
http://www.amucontrollerexams.com/courses.html

If you want to do CA along with BBA then you have prepare yourself for the CPT exam first and then you will go to the IPCC Level by clearing CPT but if you want to enter after BBA then you can directly get admission to the CA IPCC course.

Yes, NIFT allows student with the following qualifications-
The +2 level examination in the 10+2 pattern of examination of any recognized Central/
State Board of Secondary Examination, such as Central Board of Secondary Education /
State Board New Delhi, and Council for Indian School of Certificate Exa

Selection of the college is totally your choice. You need to check if you meet the minority or sports quota eligibility critreia.

There are a lot of courses available for commerce students. You can do CA, CS or CMA. Else you can go in taxation field. Even the share market is a budding job field for commerce aspirants.

Take some certification like financial risk management or financial analyst.