CBSE 12th

Yes, NIOS certificate is valid in Nepal. NIOS is flexible to study; it is recognised around the world.

Go ahead according to your interest. You haven't mentioned here your stream. There are many higher career education opportunities. A good career depends on your hard work and sacrifice.

Give the 12th exam once again. Don't worry, failures and success are the part of life. People do diploma for average jobs like 25,000 per month or even less than that. You can do diploma but better to give 12th again and move ahead in life instead of going back. Don't underestimate yourself and keep yourself confident. Life is full of ups and downs. If you have any doubt, feel free to comment. But don't take any decision without proper analysis

Your 10th marks are good. Your 12th marks are on the lower side. Your UG CGPA is also average. Any non-
managerial work-ex, specifically IT is not relevant work-ex for MBA. You will get some points for your work-ex however. Anyways, if you are a General Engineer, you will need to target 99.9+ percentile in CAT. Also, don't restrict your options to just IIMs. There are some amazing non-IIM Schools also in India that you could apply to - FMS, MDI, IMTG, SP Jain, XLRI, XIMB, JBIMS, NITIE, IIFT NMIMS, SIBM, etc.

You can go and study abroad but your marksheet will show that you have reappeared for the exam so it may affect your admission in foreign universities.

Yes, it's compulsory to get at least 65% in 12th.

You can do so but you won't get a college. You can't even get admission in a private college too.

There is many colleges in Bhopal you can get. Some of them are SIST, OIST and LNCT2. If you want to know that which college that you can get on the basis of your rank, then click on the link given below and watch the video till the end:
https://youtu.be/3YAzEP1vgvQ
https://youtu.be/hOlit6Un7l0

Entrance Test
Engineering Agricultural and Medical Common Entrance Test, commonly called as EAMCET, is an entrance exam required for admission to some engineering and medical colleges in the states of Andhra Pradesh and Telangana, India. Many other colleges offering graduation courses like BDS B.Pharma B.Sc. Pharma. D, B.E. / B.Tech etc. Require qualification in this exam.

If you are from general category, then it is very low chance to get admit in this course, because the cut off of this course closed in above 90%. But, you had a little chance in out of campuses colleges.
If you belong to OBC category, then you must get admission for this course. Mostly in last 3-4 cut off, you will get admit for this course.
For more information regarding admission process, visit DU website.