You have a pretty strong profile.
The only potential hurdle is the GMAT -- even with a 680 you won't be out of the running entirely (it helps that you're already at Columbia - the b-school will know that you won't be applying anywhere else so that if they admit you, they know you'll matriculate).
All you really need to do is to re-take the GMAT and hope that it's higher than 680 - if so, great. If not, just apply anyways and do the best you can on the applications and interview. From what you wrote, it seems like you have a solid story *and* a credible/realistic one -- you are the kind of candidate that is ideal for a JD/MBA program (i.e. an aspiring lawyer who wants a business foundation).
The other thing to do is to see if you can meet with any of the Columbia b-school adcoms -- get them to remember who you are since you're not the typical MBA applicant, but a student of the university. You may even be able to meet up with them informally during the summer if they're not too busy, or at a school event, etc.
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