B.Com

Hi Sweety,
In B.Ed, there is 2 optional subject and 5 compulsory subjects and the 2 optional subjects are based on your graduation stream. So, you can choose as per your requirement. You can take maths or English as optional subjects.

Yes, but you have to apply to change application and that is only done in March/ April admission period, irrespective of the college and or university. So, the loss or the disadvantage is that you waste a year, however do something that you are passionate about, even if an year gets wasted, instead of repenting later of having a degree, career that is actually not your cup of tea, and then no zeal of getting into it. Talk to your university official clearly about it, and they will surely guide.
Wish you luck.

The eligibility for Bachelor of Commerce:
The basic eligibility for admission to any of the B.Com programs is mentioned below:
Candidates must clear Higher Secondary School examination from a recognized educational board.
They should have Maths and English as core subjects.
Minimum 50% marks are required at 10+2 level.
Although, candidates from Science and Arts stream are also eligible to apply, those with commerce stream are preferred.
There is no work experience required for the course. However, students with work experience can also apply.

Yes. Just make sure your PUC is from a recognized board which TNOU is ready to accept.

Yes, but the minimum percentage to qualify for MBA at IGNOU is 55%.

After completing B.Com you can go for higher studies. You can go for courses like CA, M.Com and MBA in finance.
The below-mentioned link will give a better understanding of exactly what options you can explore after B.Com:
Link: https://www.shiksha.com/accounting-commerce/articles/course-options-after-b-com-blogId-12939.