CBSE 10th

The eligibility depends on the type of course that you wish to pursue, the details of some courses are stated below:
Diploma class 10th pass from a recognised board with minimum 40% marks.
Bachelor's Degree: Class 10+2 examination with at least 50% marks.
Post-graduation degree: Bachelor's degree from a recognised university with at least 45% marks. Graduation and PG programs may also require you to appear in the entrance exams.

https://www.shiksha.com/college/indian-institute-of-management-Indore/exams/ipmat
You can get all the necessary details from the above link.

To study in UK it is important that a foreign student should have completed his/her 12th in his/her own country from a recognised board and after studying in London it is important to appear for IELTS or GRE.

There are a lot many options ahead. Don't choose on the basis of what is currently in demand or what promises to do so. Find what you want to do or rather what interests you the most and then choose the path accordingly. You have a lot many options right now and are prone to feeling overwhelmed. Seek your elders' advice, you don't have to follow them.

Yes, you are eligible for scholarship, but universities will assess your profile and then they will give you a scholarship.

Marginal means just pass. That is, marks which is very close to the lower limit for passing.

I think you may try but chances are less because most of the universities/institutes require that the candidates should have at least five years of experience as an executive or an administrator in a commercial/government establishment/registered organization after passing the Bachelor's degree examination.
Best of luck.

Am I eligible to get admission for mechanical engineering in Diploma. I have scored 79% in SSC board exam.

Hi Sarthak,
Good to see this Ambition. Best wishes. You need to be very hard working for the next 5 to 6 years. Take Arts or Commerce and develop real. Good control and command over your General. Awareness. Do 12th with Political Science, Economics and Geography areas. Then A Graduation with Political Science, Public Administration and Geography Economics etc. Along with Graduation keep your Prem great. Take Professional Guidance or Coaching in the second half of Graduation and after graduation as well if needed.