Diploma

Hi Christeen,
Your eligibility for IIMs depends upon the percentile you score in CAT and your academics.

You can pursue online courses from various education providers such as coursera, edX. Refer to the links below:
https://www.coursera.org/learn/market-research
http://www.tru.ca/distance/courses/mktg3471.html
https://www.edx.org/course/buyer-behaviour-analysis-curtinx-mkt3x

You may check:
https://goo.gl/h7xdCL

There is no comparison, there are two very different fields.
You can check the details: https://www.shiksha.com/medicine-health-sciences/paramedical/course/diploma-in-operation-theatre-techniques-dott-jamia-hamdard-university-hamdard-nagar-delhi-84756

There is tremendous scope in tourism and travel management as you can see, each state has come up with their own unique ads describing the beauty of their respective states. You can develop apps helping tourists and Government in managing travel and tourism in each state and the whole country. Thus,

I would suggest you go for a degree course in the field of automobile designing. the best colleges to get into:
1. Industrial Design Centre, IIT Bombay
2. National Institute of Design, Ahmedabad
3. Department of Design, IIT Guwahati
4. Indian Institute of Technology, Delhi

Hi Kintan,
Since, you already have a Diploma and also a B.Tech degree, you cannot do another Diploma, but you can check for PG Diploma or graduate certificate courses. You must meet entry requirements which are different for different courses and colleges.