Colleges in Kolkata

Here is the complete information about ISI Kolkata:
https://www.shiksha.com/college/indian-statistical-institute-Kolkata-baranagar-19349

There's no such guarantee. It all depends on your performance and skills.

It changes based on multiple factors but generally you would have to spend atleast 5 hrs daily.

Eligibility criteria:
Students with 60% aggregate marks (or an equivalent grade point average) in Bachelors degree in any branch of Science, Engineering, or Medicine are eligible to be apply for admission. Students from the SC, ST, OBC (non-creamy layer), and PWD categories shall be given a relaxation of 5% marks.

Yes, you are eligible for Masters Degree in Social Work.
A candidate who has passed B.S.W. / B.A. degree examination fromunder a recognised university can see MSW admission. A candidate who has secured minimum of 50% aggregate marks in B.S.W. / B.A. degree under a recognised university has the required MSW course eligibility.

You have to clear all the subjects only then you will be able to pursue M.com.

Techno India University offers Integrated M.Sc-Ph.D in Microbiology. It is a 5 years intensive programme for students who have cleared full-time B.Sc. Or its equivalent examination.