Thanks for posting. So you've obviously got quant and technical skills, and likely analytical thinking etc. Your GMAT is solid, and your mentoring and tutoring commitments show dedication and a sense of giving back to the community and caring about others. Your GPA is going to be below average at strong programs.
I think the biggest challenge is explaining your overall story-- how your past experiences brought you where you are today; why you need the MBA; and what you'll do with it in the future. Some of this you can't change (what you've done in the past), but some of it can be influenced or improved by how you explain your story and truthfully, in your essays first of all (as well as resume for your prior experience). So I would definitely spend time working on that resume, your essays, and your overall explanation of who you are, why you want the MBA, and what you'll do with it. To change to marketing, it would help to show you've had some experience and involvement in that area already; sounds like you did have some but based on your explanation here I'm a little unclear on that, so I'd encourage you to highlight that further.
As for schools, it's likely that Kellogg and Columbia will be the biggest reaches on your list; you may want to look outside top 10-15 for some additional options to give you better overall odds as well. Continue to research, let us know how it goes and any other q's!
mbaMission Insiders Guides: http://www.mbamission.com/guides.php?category=insiders
Free Consultation: http://www.mbamission.com/consult.php