B.Com

Two things, one the answer by Partha - tough to shift from commerce to Science. So, probably an MBA or an MS in Finance are options, but not MS in the Science stream. Second, US typically requires a 4 year UG education. B.Com is only for three years, and so 1 year M.Com may be needed.

Yes, you can do Diploma in HRM.

This is depend on the city you live. If you live in Kolkata, then you can select Adrews College & Basanti Devi College, both are good colleges.

As comparison.bba ,b.com is right course.

Please check for your eligibility to pursue B.Com from Ramanujan College at the link provided below:
https://www.shiksha.com/accounting-commerce/course/b-com-ramanujan-college-kalkaji-delhi-259032

Yes, you will be.

Hi Suryabhan,
It is although not allowed, but some colleges might give this privilege to change the stream. You will have to talk to the concerned authorities of your college about the same. They will be able to guide you well in this regard.

Hi Usha!
In order to pursue a regular course, you will need the following documents for admission:
class 10 certificate
class 12 certificate
character certificate
Migration certificate (if needed)
An ID card like Adhaar, Voter ID card, PAN card etc.