CBSE 10th

Since getting into IIMs is for a class which has millions of students, the average student is probably a 6 pointer. Well, 15% of IIM Calcutta Batch of 2019 are 6 pointers or worse. So, yes, IIM Calcutta has a lot of average students considering only academics. But, an average student cannot make it

https://www.shiksha.com/mba/cmat-college-predictor
Here is the tool which you can use to get a good prediction about the college according to your score range.

Your marks are pretty good, the only problem is with graduation percentage since it can reduce your chances to get admission in a reputed MBA college. You can appear for MAT and also secure admission in a B-School but just make sure you can justify the valid reason for graduation marks/percentage. G

Digital design is not explanatory. Presently most of the leading Institutes like IIT-Bombay, IIT-Hyderabad, IIT-Guwahati, NIFT, and NID are offering the following B. Des courses after 12th as under.
1. Animation and film design
2. Apparel Design
3. Ceramic and Glass Design
4. Digital Game Design
5. Film

No, it is based on 12th.

It seems to be difficult.

Hi
Yes, the cut off of CMAT is 80. Then you are eligible to get a seat in Welingkar Mumbai. You can use these links also:
https://www.shiksha.com/mba/cmat-exam-results
https://www.shiksha.com/mba/cmat-exam-cutoff

You can take UPSC exam, because mininum eligibility is, Bachelor's degree from a recognized university, even if the degree is through distance.

Yes, you can do any Diploma after your class 10th .

Yes, you are eligible.