IIM

As far as Great Lakes is concerned, you have slightly dim chances.
All the best.

There is no specific eligibility criteria for class 10th and 12th percentage, there are just ratings associated to these scores. So, it is recommended that you aim for a high percentile to make it to a good IIM so that your ranking based on a weighted average score is good.
I cannot pinpoint to a specific score because it is all relative, but you should aim for a percentile >99.

The current fee amount is Rs. 21 lakhs for two years, which includes tuition and accommodation (excluding food expenses).

Yes, but you should be very clear about your thought process during the interview.

There are a lot of good MBA colleges in India some of the colleges have more rating than some IIMs. Colleges like IIFT-Delhi, Jamanalal Bajaj-Mumbai, XLRI-Jamshedpur, IIFT-Mumbai, ISB-Hyderabad, and many more. Do check the list of top MBA colleges in India on Shiksha.com, you will get a good quality of information from Shiksha. Their rating scheme is good and they mainly focus on placements.

Yes, because yours is the government approved degree and you have all the things required for MBA. IIM require good marks in CAT for eligibility if you can score those marks then you will be called directly. So don't stick with the stigma if you have any and start studying.

Yes, you can apply for Ph.D after completing your PGDM. But it's always better to go for a full-time course if you are not an entrepreneur and have a lot of free time. You get to learn much.

No course. But, you should prepare for the CAT exam which is required to get admission in all the IIMs.

The CAT cut-off at Ahmadabad is always around 99.5 pr.