Short term

No, it is not offering any short term Diploma course.
Refer to the college link for details: http://rizvihmct.com/

You can work on speaking skills courses. If you have interest in Analyst and Research kind of role than you can do a short course on Advanced Excel, R/Python or Big Data. If you are looking for Marketing Research kind of role than SPSS is a good tool to have hands on. Just look for the role you will be aiming during your MBA for summer/final placements. What kind of work suits you, do a bit of research on that and select a course accordingly.

In 20 days, you can only brush up the basic part in Java, learning any language completely is not possible in just 20 days, but I would recommend you to go ahead and learn as much as you can in 20 days and then sneak out few hours on a weekly basis to master Java.

Hello Raquib,
If you are going to join TCS in a month's time, I would strongly advise you to learn something new that can relax you. Like singing, playing an instrument, learning to play badminton or squash or swimming, etc. This will come in handy during high-pressure job in TCS.

Postgraduate courses in securities management, quantitative finance, risk and treasury management and capital markets are all available in National Institute of Securities Markets (NISM).
Similar courses are also dispensed by CCGRT (Mumbai). On the same note, certifications and licenses are also given by BSE and NSE relating to compliances.
You can always go for another undergrad degree or a Masters degree. For instance, a student who has done B.Com enroll for M. Com or LLB, or the ones who have completed LLB enroll for LLM.

There are various certificate courses available that will also match up to the studies done in M. Com:
Certificate in Banking
Certificate in Stock Market
Certificate in Derivatives
Certificate in Income Tax
Certificate in Accounting
Certificate in Micro Finance
Certificate in Investment Banking

Please visit NIBM Global's (National Institute of Business Management) official website for online MBA training and certification. NIBM offers online training and certification for one year MBA, two year MBA and one year EMBA. Please note that NIBM does not offer an MBA degree, but it serves as a study aid. NIBM Global MBA training programs are suitable for working professionals and MBA aspirants.