Hi there,
Thanks for posting and welcome to GMAT Club!
You've shared a lot of info here, thanks for the detail. I love all of your international experiences. And working in the mining industry is a bit less common so I'd emphasize the experiences and perspective you bring to the table because of that. That said, there are a LOT of Indian engineering applicants at b-schools these days so it is quite competitive. Your application and GMAT and essays can really make or break the situation.
Re: the GMAT, well you want to score as high as you possibly can!!
I recommend a prep course such as
Manhattan GMAT or Kaplan, fyi, and plenty of full-length realistic adaptive practice exams. You can view the average scores by school easily on their website or the US News ranking list, many of the schools you mentioned have averages of 710-720. You mentioned scholarships... just know that they are rare and reserved for the very top applicants, and often given in Round 1.
You have time to apply for Round 2 depending on how much time you can devote to GMAT and essays and all that.
As for volunteering, it would help... try to pick an organization or activity that you feel you can speak more passionately about if possible.
Re: schools, you have listed some very competitive schools... they could be a bit of a reach, depending to a certain degree how things go on the GMAT, with Duke being most reasonable... do some research into other options as well to be more prepared. For more on choosing schools, check out our free download at:
https://info.mbamission.com/Selecting-Yo ... ram-E-Book.
Let us know if you have further q's and good luck!