B.Sc

Taking up tuitions is something that you need to decide. If the faculty members are great in the college in which you enrol, then I don't think you would need any extra classes.

It is not mandatory that you must have a BCA degree, you can do MCA without BCA.

Stats from both Ramjas and KMC is good. I can say the same for Economics hons for Ramjas. Khalsa had a reputation for violence a few years which has improved and considering the opinions of my peers from Khalsa, it is a satisfactory college. Also, as per my knowledge, English and Economics faculty is damn good there. So, it's your call now.

As much as I have heard from people from the science department as well as other people, Ramjas has a good science faculty, however the same goes for KMC. The opinions based on this question is very subjective. Lot of people consider Ramjas's science faculty to be better, and others consider KMC to be better. So if you choose either, you'd be in an appropriate college for the course you want to pursue.

If you are looking for career in business domain of IT industry and aspiring for roles like Analyst and Consultant than BBA is better options. If you are looking for roles like Tester, Developer, and Team Lead than B.Sc. Is better. B.Sc. Will give you a technical view and over the years as you gain domain knowledge you will be suitable for a good techno-functional role and it will be easy to get good roles in top IT MNCs.

If you have done B.Sc in food science, then yes, you can apply for M.Sc nutrition and dietetics from Amity University Noida.
Yes, you can easily apply as the aggregate score is 50% in graduation.